This is from my limited knowledge as a user, I don't work for Aternos but I've had an Aternos account since 2014 and have picked up some useful info that might help.
A lot of what you seem so be asking for is really difficult to implement the way Aternos has to for security and as much as they can has already been implemented in the best way they can.
Most premium servers Allow FTP access, with this you can do just about anything (including things you really should do like uploading a bitcoin miner for instance) but they can afford that risk because you're paying more for the hardware than it'd cost if you did something you shouldn't, that and you have an allotted "amount" of server that you are sold, it would be difficult to exceed that limit.
Now here's where Aternos is different, they have a limited amount of hardware (whatever their ad money can afford) and FAR more users than they can support with their hardware all at once, not only that, but every user is equal, there's no premium plans so they have to squeeze the room for your server from their limited hardware
The TL:DR point is that if they have to spend more money on security, they have less for hardware, so if they can rely on outside sources ( like curseforge ) then that money they save goes on hardware and a better experience for the most users possible, but sadly that means that they have to Sacrifice the features that would give us the most customisation like FTP access, because sadly such features are too easily abused by potential wrongdoers