Re: True in every way in fact
Can the EU mandate that european businesses cannot use Salesforce?
No, but they can say that (using your example) Salesforce doesn't comply with the law and hence it's illegal to use them. The reason we don't have the big homegrown tech is that the US has provided an environment where they could get going and through a variety of illegal techniques killed off any meaningful competition.
If the law was properly enforced such that use of "US services" was effectively illegal, then we'd rapidly see a number of options pop us -and some of them already exist.
Take MS. In theory* we are told that the data centres in Ireland are operated by a a separate business resident in and subject to the law in Ireland. In theory*, the us corporation known as Microsoft is physically unable to access data held in a data centre in Ireland. I suspect some of the others have already set up such structures - and if done right can comply with the law.
Any of the usual suspect will be able to sit back and either sort out something similar, or see their EU business dry up - and as pointed out, the EU is big enough than few of these international corporations can afford to walk away from it. But if they did, then others will be happy to pop up and fill the gap.
* I say "in theory" because it's not as clear cut as they claim. Firstly, the domain names used are under US control - so there's no guarantee that things couldn't be redirected for nefarious purposes. We've seen how this complex international web of stuff can create fragility where the failure of a server somewhere can cause outages for customers on a different continent. Secondly there's that rather inconvenient issue that MS in the US handed over data held on Irish soil the day after the US passed the CLOUD act.