I think what a lot of people are missing is the very likely fact that AoE2:DE was not really made at all to succeed long-term as a standalone product. For example, AoE1:DE came out just recently, but has effectively stopped being supported (despite there being some long-standing issues). AoE3:DE is also coming out shortly. There is no real reason to suspect that AoE2:DE won't slowly lose support as well.
The reason is that - and this is, of course, just my opinion! - all of the Definitive Editions, for AoE1, AoE2, and AoE3, fall into what is effectively a marketing push for AoE4. "Age of Empires" might have a huge reputation for us (relative) oldies, but I suspect that younger gamers probably can't recognize it as immediately as you'd expect, and even those who played it as a kid may have forgotten about it since. However, by releasing a 'definitive edition' of every entry in the series, this brings the name back into the spotlight, the games get media attention, people start talking about it online, big name streamers show the games off, etc. Sure, Microsoft of course wants these games to be profitable in their own right, but the major concern is making the franchise, 'Age of Empires', as valuable as possible before AoE4 releases. Whether AoE2:DE has a thriving online scene really isn't relevant to that.
Now, the response to that is likely going to be, "Well, if AoE2:DE isn't supported, then their reputation, as well as the Age of Empires brand, will be hurt". This is a fair comment, and I've heard a few people bring it up. However, consider the shambles that was AoE2:HD. Did that stop everyone from buying DE? Or the lack of support for AoE1:DE. Did that stop everyone from buying DE either? Given that, do you really think that a lack of support for DE will stop people buying AoE:4?
tl;dr: In the short term, AoE2:DE serves to make money off of the people who buy it on release and forget about it (as ZeroEmpires said earlier). In the long term, AoE2:DE serves to bring the name 'Age of Empires' back into the spotlight in preparation for Age of Empires 4, and so doesn't require long-term support.
The reason is that - and this is, of course, just my opinion! - all of the Definitive Editions, for AoE1, AoE2, and AoE3, fall into what is effectively a marketing push for AoE4. "Age of Empires" might have a huge reputation for us (relative) oldies, but I suspect that younger gamers probably can't recognize it as immediately as you'd expect, and even those who played it as a kid may have forgotten about it since. However, by releasing a 'definitive edition' of every entry in the series, this brings the name back into the spotlight, the games get media attention, people start talking about it online, big name streamers show the games off, etc. Sure, Microsoft of course wants these games to be profitable in their own right, but the major concern is making the franchise, 'Age of Empires', as valuable as possible before AoE4 releases. Whether AoE2:DE has a thriving online scene really isn't relevant to that.
Now, the response to that is likely going to be, "Well, if AoE2:DE isn't supported, then their reputation, as well as the Age of Empires brand, will be hurt". This is a fair comment, and I've heard a few people bring it up. However, consider the shambles that was AoE2:HD. Did that stop everyone from buying DE? Or the lack of support for AoE1:DE. Did that stop everyone from buying DE either? Given that, do you really think that a lack of support for DE will stop people buying AoE:4?
tl;dr: In the short term, AoE2:DE serves to make money off of the people who buy it on release and forget about it (as ZeroEmpires said earlier). In the long term, AoE2:DE serves to bring the name 'Age of Empires' back into the spotlight in preparation for Age of Empires 4, and so doesn't require long-term support.