Would $7k be better spent on a new-er used big white? Or a kahuna body like this? If I had that kind of money once in my life, which way would I go? I'll have some fun pondering that.
I think your question is just posed as a fun thought experiment, but someone with funds for only one or the other should in fact put in their retirement savings instead. People live a very long time these days and you'll need the money.
Otherwise, of course it's a question of what you find yourself wanting or doing that your current outfit doesn't let you do. If you don't have 100 photos with a 100-500/4.5-7.1 all at the long end that you think would be far more valuable commercially or to yourself if they had far less DOF or a lower ISO, then a 600/4 would fulfill an actual need. If you've never found a problem with a lack of global shutter, then global shutter wouldn't be a selling point.
There's also the question of actual cost of ownership. This equipment doesn't cost the purchase price, it costs the purchase price minus the sale price. If you find yourself needing a 150mm aperture's bokeh, then get a 600/4, but get for instance the EF 600/4 IS MkI, which isn't significantly worse imagery than the newer ones, but can be had for $4000, far less than its new price and even farther less than the RF model. The EF600 isn't going to drop in price at this point, which means after 1, 5, 10 years you'll be able to sell it for what you paid, and I'd even expect it'd go up roughly in line with inflation. It's main shortcoming is weight, but you can shoot a EF MkI basically for free. So try it, and only if you find you just can't shoot it due to the weight should you then consider trading up. So back to your thought experiment: you're debating a big white, which can be had basically free, vs. a brand new piece of consumer electronics, whose value will drop 90% or more in 15 years. So nothing against the R1, but simply due to its novelty and ensured quick obsolescence, it would not be my choice.