The answer will vary for each person responding. The theme of moving up is not uncommon here, but it is far, far from the norm. Often it is folks first getting out camping, some for the first time in a trailer at all, let alone an Escape that may be smaller than they were used to, and while gaining experience also finding out what may work better for them. Many realize the 19 truly is best for them after time put in with the 19, or the bit more room in a 21 works better, or even more room in a 5.0 is the ticket for them.
When we bought the 19, it was the biggest Escape at the time, in fact it was brand new, and we were in on the introductory offer. It gave us everything we needed in a trailer, and in fact it still does. It provides lots of storage for our needs, and this is without the storage box. For now it is all we want or need, as we must use a bumper pull, and the 21 layout does not suit our style at all.
We do have a bit of a yearning (as displayed in my sig line), and the only way this will be a go, is if I first retire, and sell off the two vehicles I currently tow with. The draw to it, is that it has a queen sized bed, that is out of the way, which is the only real reason I don't prefer the 21 myself. Like Dave mentioned, even though longer itself, when towing overall it wouldn't be. Towablility may be a bit better, but my 19 tows like a charm, so that is not really a consideration. It does have more inside storage than the 19 or 21, but there is no capacity to add a storage box if one was desired. It also means using a pickup, which lessens the amount stored inside with an SUV, or even a pickup with a topper. Plus, it is going to be tougher to bring our canoe along, something we would have to find a solution for before purchasing.
Part of the thought of me getting a new Escape at all, is the possibility to outfit it with the options I want for retirement. Heck, at least one couple have bought the same model (the Eddys with a 19) so they could do just this themselves. But who knows, I don't that's for sure, what I will end up doing, I keep reminding myself that the 19 does truly work very well for us, and would likely do so into the future as well with just a few upgrades.