a) absolutely - they are not stopping you from doing whatever you want on your machine, they are just not allowing to use their service. I don't think anyone would advocate making "ad blocker" a protected characteristic alongside race, gender, sexuality etc

b) absolutely - most people either don't use ad blockers, and of those that do, most people are not militantly committed to them. There will be a set of people who stop using the service out of principle, but almost all of them weren't making any revenue for spotify anyway.

The only risk to spotify I can see is if they use somekind of nefarious method to detect ad-blockers (eg root kits) and I 1) don't think they're that stupid 2) don't think they would be necessary or useful anyway.

I guess another risk is if they force people to turn off ad blockers, then accidentally serve them malware, but afaik, various other sites have done this and nobody seems to care.