My rule of thumb is that one can improve five years significantly - so better you are where you like to be then.
I did not think about it that way, but that sounds about right. After five years most have an idea what their dance looks like. It might not be exactly "improve" thought- it is more that by that time the direction of ones development is mostly fixed, and there are not going to be any massive changes in approach anymore, just refinement. (though there are some people who completely change direction - but that seems to be usually prompted by some outside event, or by some options/ideas opening up that were not available before).
I don't really see that once they reach that stage the skills of intermediate dancers deteriorate if they don't keep taking classes - there is sometimes a bit of drift when different things/approaches become fashionable, but nothing that some mindfulness can't fix.
In terms of how i think of development of ones skill i think the question of "good enough" is in some sense the wrong question. An idea that i have stolen from my wife who is into yoga is the idea of an activity being a "practice". These are skills that are self sustaining, and doing them longer will lead automatically to refinement and changes of ones practice, but this refinement is to some extent orthogonal to what the activity is about. If one does yoga longer one gets better balance, and flexibility, and the ability to do tougher figures and so on, but there is not really a point of "good enough" - and it is really difficult to imagine what that would even look like: would you stop doing the sun salute every morning once one is good enough? Do it with less mindfulness? Do the pose so that it is no longer actually a stretch?
Tango is for me similar - it only exists on the social dancefloor, in the dialogue (multilogue?

) with the ronda, the music, and the partner, and by having this dialogue (with some minimum level of self awareness) we will over time get better at expressing ourselves and (maybe more importantly) listening. We can practice specific skills that can scaffold and facilitate this conversation, but in the end it is only the milonga that teaches tango. Basically i think a lot of the discussions we are having about skills and vocabulary are more about preparing oneself for dancing tango than actually about dancing tango - just like e.g working with weights and doing other stretches and balance exercises and learning different poses will support ones yoga practice, but are not actually doing yoga.