It looks like you've found yourself in perfect learning situation. Currently your application works, and I'm sure you're very familiar with all of the 20k lines of code. Making changes isn't that difficult, and, so far, there doesn't seem to be any reason to change your development approach.
You are a cowboy! And being a cowboy is a lot of fun!
But being a cowboy is a lot of work, and even the best cowboys run out of steam after a while (sorry about the mixed metaphors).
In the question you mentioned that the app will be used and maintained for years to come. Those 20k lines will surely increase, and with every new feature you add, and every new bug you squish, you'll notice that it gets harder and harder to write code without breaking existing functionality. This is tightly coupled spaghetti hell!
MVVM as a pattern encourages developers to loosely couple their code. In the MVVM world you keep your Model separate from your view state (View Model) and display logic (View). This does require a bit more up-front work, but the downstream benefits are many:
- Unit testing your Model is easier. No View baloney to worry about.
- Integration testing your View Model is a breeze
- Because your M, VM, and V are distinct sections of code, modifying one has a near 0% chance of causing regressions in the others.
- And there are at least 7 other reasons.
But the most important reason to switch to MVVM over CBV (Cowboy View) is this: You're human. While humans are actually pretty good at creating mental models of complex things like software, doing so is a very draining endeavour. And keeping track of frequently changing and ever growing software is a recipe for mental illness.
We are flawed sacks of flesh. But fret not, for we are also organised flawed sacks of flesh.
By organising your code into classes with single responsibilities (eg, a ViewModel) you can now work on subsets of your application without having to load up your mental model of the entire application. Your brain will thank you for this!
But don't take my word for it. Like I said at the beginning of this pseudo-rant: You're in a perfect learning situation. You've made a classic mistake (there's no shame in that) and you're going to be around long enough for it to catch up to you.
Switch to MVVM now if you want. Or don't. Either way, you're going to eventually learn why it's a useful pattern.
My recommendation: try to maintain your cowboy spaghetti for as long as you can. The best way to learn is by making mistakes. So make your big ones early and often!