A lot of ISPs will not give out their configuration options and handshake credentials to force you to use their provided equipment only.

Not exactly, for some ISP's, internet only works with Fitzbox. For other modems, you need to go through months worth of to and fro with ISP to even get them working

FritzBoxes are used as voip server/bpx, printer server, nas, and home automation. And probably more. (German) telcos provide specific FritzBox configuation instructions (select correct sub-branch of provider and contract kind), adding the quest to get real connection data to the attempt to migrate to anything standardized.

Switching to something else is not as easy as buying a new modem for most normal people.