Also, having to retrain users to use a new shitty UI after they got used to the previous shitty UI is a major moat of many SaaS services. The user doesn't care about the web portal, they just want to get work done. Switching to a different web portal needs to be a big net positive because users will correctly complain that now they are unproductive for a while because the quirks and bugs of the previous SaaS don't match those of the new SaaS.
In a world where the interface is "you talk to the computer" you will be able to swap providers way more easily.