They have no right to delete installed games, do they? How, exactly, is steam going to prevent you from accessing the files on your computer? They are always there and ready to be archived by you and even Steam itself.
Now, some, or even many, new games may refuse if they are somehow extremely dependent on the service. The ones that I have tried run fine. Most old stuff, though.
> How, exactly, is steam going to prevent you from accessing the files on your computer
The vast supermajority of games on steam require steam itself to be operable in order to even run. Valve absolutely can break or turn off steam and (legally!) revoke access to most games on most steam user PCs that way.
Now, some, or even many, new games may refuse if they are somehow extremely dependent on the service. The ones that I have tried run fine. Most old stuff, though.