I'd say all the blobs that keep us from moving to a random modern GNU/Linux distro while keeping the core functionality are a dead giveaway.

Of course, because the aging hardware won't be able to keep up with the increasing demands of its users.
But that's not the (only) problem with the N900/Maemo. The real problem is, that we can't actually keep Maemo up to date or move on to a newer OS entirely.

Im still running a desktop PC from 1998 on Debian Jessie for fun that has about half the computing power of an N900 at stock frequency.
I'm probably going to retire it once Jessie runs out of support because it only has an i586 CPU which is not supported by Debian's kernel anymore and frankly I'm to lazy to actually learn how to properly run BSD just for fun.
But by that time the PC will be more than 20 years old and I would not have kept it so long had I been stuck with the stock Win98. That's the difference to the N900. I can't just switch to another OS without losing phone functionality, which I consider kind of essential on a - well - smartPHONE.