As long as the subduction zones along the pacific and north american plates remains active so will the cascades. Just look at the number of volcanoes along that area and you will see another will blow like St Helens, maybe even bigger.

The process of eruption and rebuild is a constant one, albeit over a long period for human recollection.

They are slowly drifting.  Some cones putter out, as the North Sister seems to have, but the South sister (the youngest volcano of the Three Sisters) developed some ground deformation -- specifically an uplift over a broad area -- starting in 1997 that might be an early precursor to an eruption.

Ummm, Rainier is central Washington.  It wouldn't affect anyone in Canada any more than St. Helens did.  In point of fact, its only 50 miles north-northeast of St. Helens.  And larger doesn't necessarily mean more destructive ... Rainier is an older peak, so something could be said for it having matured to build a cone of its size.  St. Helens was actually not that much shorter than Rainier before she blew.

The Juan de Fuca plate is sliding under the North American plate, not butting against it, there's a subduction zone along the coast, and as it grinds under the North American plate, it creates heat and that's what fuels the volcanic chain a couple hundred miles east of the subduction zone.

Los Angeles is on the Pacific plate (or the cusp between the Pacific and North American plates), not the Juan de Fuca; the Juan de Fuca plate does not extend farther south than northern California (and Los Angeles is southern California).  The Juan de Fuca plate is sliding northeastward under the North American plate, not northwest (the North American plate is sliding north-northwestward relative to the Pacific plate).  The Juan de Fuca plate (which is actually fractured into three plates -- the Explorer plate to the north, the Juan de Fuca name kept for the central piece and the Gordo plate to the south) is believed to be one of several small remants of a once massive oceanic plate called the Fallon plate that predated the Pacific plate, but by now the vast majority of the Fallon plate has subducted under the North American plate.  Other remnants of the Fallon plate include the Cocos and Nazca plates.  

I was watching a video documentary about USA national parks.  Spooky, eh?

Seattle is about as far from Mount Rainier as Mt. Rainier is from Mount St. Helens, about twice as far as the kill zone from the St. Helens blast (referring to the blast itself).  The sideways eruption also seems to be rather rare (in fact, unheard of by geologists prior to St. Helens).  The sound of the blast can go quite far (St. Helens' eruptive blast was heard in British Columbia, Canada), but the actual searing shock wave from St. Helens 'only' went about twenty miles (twenty miles is plenty huge for something travelling at the speed of sound killing absolutely everything, of course).  If Rainier produced giant mudflows, to the extremes it has in millennia past, it could potentially push a mudflow through 'the Valley' (a valley with no well-known name as it is not a river valley through which State Route 167 goes and includes the cities of Sumner, Auburn and Kent with Renton capping its northernmost extent) to Renton and into Lake Washington, which borders Seattle's eastern edge, but even if that generated flooding and/or tsunami on Lake Washington, Seattle would have relatively minor direct effects aside from the Rainier Beach area as much of Seattle is on a hill between Lake Washington and Puget Sound.  The most likely effects Seattle would incur would be ashfall and I-5 getting severed (the major north-south interstate of the West Coast that goes from the Canadian border to the Mexican border, and connects Seattle and Los Angeles) by the bridge over the Cowlitz River getting potentially washed out and/or flooded and choked with debris, as well as Seatac and all other area airports being shut down due to ash in the air.  We've had some debilitating isolations a few times in the past when I-5 was closed down due to flooding on the Cowlitz to the south, and I-90 to the east closed due to mudslides or avalanches from the same heavy precipitation that caused the Cowlitz to flood, effectively severing all land transportation routes from the Seattle area.  Rainier erupting could do all that plus shut down air travel, plus much of our regional power comes from local hydroelectric dams such as the one on the Puyallup River which could be destroyed by even a moderate lahar on the Puyallup river.

I currently live about 28 miles from Rainier a bit southwest of Orting on that map, as the crow flies, but on an exposed hill, so there would probably be some effect here ... though I'll likely be moving soon to someplace that, sadly, has less of a view but is far more exposed to a lahar along the Puyallup river.

Good old Yellowstone - that's due for a pop sometime soon

If by 'soon,' you mean several hundred times longer than all of human-recorded history thus far.  Yellowstone isn't close to 'due,' its well below its average span between mega-eruptions since its last eruption, though spans between super-eruptions vary greatly in duration and certainly there have been previous spans of dormancy shorter than its current period.

Things with Yellowstone could dramatically change in a very short while.  It may not appear that it could go off any time soon, but that doesn't mean to say it can't... or won't.  Sure, the science of vulcanology has come a long way in recent times, but prediticting when a volcano will erupt... or not, is still too inaccurate to place all one's eggs in the one basket.  It may well be that Yellowstone doesn't 'pop' for another 200,000 years or more, but I wouldn't want to be setting up home within its vicinity... cos it might just do the unexpected.

Volcanic eruptions are not like earthquakes, in that -- since St. Helens -- we now understand there are precursors to large eruptions.  We would likely have at least a month's warning that something SRS was about to happen at Yellowstone.  If it did happen, it would of course be very dramatic and, indeed, a large swath of the U.S. would be disrupted ... likely much of the farms in the midwest would be utterly destroyed by ashfall, rivers potentially clogged and jammed by the ash and there could be a massive 'no fly zone' due to ash in the air that could sever all air travel between the coasts ... but no, apocalyptic fantasy movies like 2012 were not at all realistic.