Have you really read it? Roma has qualified for the Champions League in 4 of the last 5 seasons,
and their numbers are still negative. If it weren't for player sales Roma would be in the same situation Milan is. I don't think you're quite grasping how difficult it is to have a 'sustainable' model that doesn't involve selling players. Every big team in Italy is operating in a similar way, only Juventus can afford keeping their players, thanks to their revenue being much higher. With the club's financial situation it is impossible to guarantee ending 2nd in the league every year, going for that would be a massive gamble that wouldn't even be worth it.
The 'sustainable' model Roma could be looking at is Napoli's, but it is worth remembering that Napoli are, in essence, a selling club as well. Napoli's finances are more sustainable right now because
their wage bill is smaller. However, it is rather obvious that by keeping wages lower you're forced to sell players once they ask for more money.
They're not ''running out of players to sell'', that's where their recruiting comes into play, and considering the players they've managed to 'find' the last few years (Salah, Alisson, Benatia, Pjanic, etc), it's clear they're doing a good job on that front.
Seriously, give it a closer look, there's a lot of information in the thread I linked, more than enough to figure out why Roma can't stop selling players. They need to increase their stable revenue, sure, but that's easier said than done and they're forced to sell in the meantime. Apparently they have plans to expand the stadium, which is good and will help their commercial growth, but that's a slow process.