By ‘older’ do you mean pre-Creative Cloud?

If so, I can't imagine what Adobe could actually do to stop them? I figure the idle legal threats seen so far (I know of two people who claim to have received emails from Adobe - I'm awaiting one myself, as I still run 2014 Photoshop on this machine) is simply so they have a platform from which to deny access to older versions as provided through CC.

Anything installed from disc is beyond their practical reach.

I'm really curious to know what's underlying this - if it is a third party reason, I'm kind of flumoxed at Adobe's stance towards its presumably quite loyal userbase!