The restriction is done trough throttling in speed.
It only looks like you have unlimited space but lets say you would actually want to upload a TB of stuff it would take very very long unless you use multiple accounts and then if you ever would need to download it it would again take very very long.
It also seems like files that are not downloaded by many people for a while get moved to slow servers and downloading these is really really really slow.
If you have a few GB and want an additional online backup sure you could abuse it for that but realistically the potential to abuse it is low as it simply makes no sense as a could storage + they simply delete accounts from people who abuse the network. "Spam" is very lose defined and if you spam upload/download I'm sure they consider that to be spam and delete your account. Kinda pointless "backup" if you can lose access the moment you use the backup.
>And they have basically no income.
How would you know? They launched ads in public channels.
>TON went nowhere.
TON exists and seems functional just doesn't have anything to do with Telegram anymore due to the SEC.