Most companies certainly won't be using "commercial grade internet" in the way that term is usually used. That would usually be reserved for large enterprises, which really only covers a small part of the workforce in practice.

Many businesses don't bother even subscribing to a business package, because something like a static IP is unnecessary for them.

Further, the point regarding VPNs still stands -- think of the chaos it would cause for many people working from home (on residential connections). And that's just one example.

I don't find it plausible for an ISP to block this.

Actually, there is "commercial grade internet" at least in my country. The main difference is that it is several times more expensive, and in the office buildings the owner doesn't allow ISPs with cheaper "residential" plans.

Business, yes, that was the word I was looking for, thanks! So the ISP could just limit the residential packages, limit the business packages to actual businesses, and that's all.