The difference at this inflection point is that as discontinued EF lenses fall out of support much of the traditional stay-in-the-system inertia evaporates.
I'm at the point that I haven't decided to go into the RF system, so if Nikon offer something compelling then I might consider trying their alternative. In 30+ years of shooting EF I've never considered that before. But given that I'll have to buy into a new lens system for the next few decades, I'll have to study the options.
In theory maybe. In practice, most people are extremely reluctant to switch systems, partly of course due to lens compatibility, but equally because they like continuity of control layout and general design. For example, the generally higher specs and arguably superior AF of Sony has sometimes tempted me to switch from Canon, and I could use my old EF glass on a Sony body perfectly satisfactorily. But, I don't like the feel/comfort/ergonomics of Sony cameras, and I don't want to miss important shots due to unfamiliarity with a new design.
What is important to me is to have the tool that works best for me, and provides the required level of performance, so before getting my R5, I looked seriously at Panasonic S1R, Sony a7Riv and Nikon Z7ii, as each offered something that I value, and the R5 to me seemed overkill, with 8K.
I ruled out Nikon simply because of lens compatibility, then I discarded the Panasonic option because of the weight and the potentially irritating "DFD wobble effect". In many ways the Sony a7Riv seemed the logical choice - lens compatibility wasn't an issue, I'm not a brand fanboy, resolution was higher than the Canon, I prefer a tilting screen, battery performance was better, price was significantly cheaper etc. But when it came to the crunch, I chose the R5, because I find Sony cameras a bit uncomfortable to hold and operate, and because having used Canons for several years I'm thoroughly familiar with them and knew that I'd be less likely to miss important shots during the "muscle memory" adjustment period.