My guess is that the 2x price increase Xs4all was charging for their plan was a bridge too far for most customers. It's important to keep in mind that the vast majority of people rent their modem, don't know or care what a /29 is, and is calling tech support because the plug is loose or the modem needs a power cycle. Bulk-blocking SMTP happened because open ports are botnet ports, and the average customer does not know how to identify and shut down zombies on their network.
[0] Assuming your provider isn't stupidly committed to "you can't have business class because you're in a residential area, WFH doesn't exist, and the zoning code is gospel, all hail Robert Moses"