The problem with mods never being seen as complete is unrelated to release schedules. It's something that most people will deal with regardless. However, generally most people have an idea of where something is "complete". Which bring us to one of the reasons why most people should completely ignore all outside input, and that is Feature Creep.
Feature creep is surprisingly difficult to manage. Knowing when to stop is an important skill, and knowing when to stop expanding concepts. Ideally, by the time a mod releases an Alpha, its features/ideas are already frozen and figured out and the developer knows exactly what he's going to be adding, when, and why, and doesn't need any further input. Otherwise he's just kind of winging it and this can spiral out of control in a hurry. Thus why mods just die out in a slow, agonizing, and pitiful manner.
Personally, I don't know any projects or project creators who have objectively finished a project they released early. Like, actually finished. Yes, this is based on personality, but most people building projects are doing it for hype/attention and are doomed to failure one way or another. Those building the projects for themselves will finish it either way, regardless if it takes years to finish it or not (for a game like sins, a TC really shouldn't take any longer than 1-2 years unless you're seriously schedule strapped or lacking in skills, in which case you're in trouble regardless). From my observations, many sins mods struggle to maintain stable production lead in the first place.
In most instances, releasing an alpha hurts you in that it will cost you motivation because you will only see another mountain ahead of you to overcome, and it hurts the fanbase because the fanbase only sees a shadow of what they hoped for lacking in virtually everything the developer hopes to create. Then they are stuck waiting for shallow incremental updates the developer has to worry about releasing instead of just sticking to his guns until he feels totally comfortable.
If you have the devotion to withstand the long crawl of incremental releases, then that's great. But it is something I will always recommend against, especially for new modders. It is harder to manage than you think, especially if your goal is indeed to reach a final release at all.
In fact, for most people, I recommend keeping public presence to an absolute minimum, limited only to research (e.g. asking for help doing stuff) and never talking about specific things until it's done. The sense of obligating oneself, either to releases or to appease others, as opposed to working for oneself's desire is a deadly enemy of motivation and personal stability. I will take my own advice and cease discussion of such matters. Let Allah decide what works for who.