Reading through this thread I'll toss in my two bits. I prefer good glass over a new camera body any day. That said, I can't justify spending money on brand new L-series lens but I am always on the look out for good condition used L-Series lenses. I set aside a little money every paycheck and build up a cache of cash. I constantly check online sites for good used lens and when I see a good price I pull the trigger.
Sigma and Tamron make some great lenses Hands down the Sigma 150mm Macro smokes the Canon 180 L. It's Probably my favorite field stacking lens. Gives you plenty of distance at 1:1 and razor sharp from corner-to-corner even on the high pixel density sensors. Likewise I'll take the new Tamron 90mm over any lens in the ~100mm focal range. Optically it's as good as any lens in this category and the price to performance ratio is off the charts , even new. There is a caveat with Sigma and Tamron though, depreciation is much greater than Canon L or Nikon Gold Ring lenses. Conversely you can great really good prices on the used
market.
Using Canon-L on a Canon R - no problem, the Canon R-EF adapter works flawlessly. I wouldn't sell L-glass, just get the adapter and your in business. Always keep good glass. You'll know when it's reached it's peak performance and that certainly isn't the case on the Canon-R.
Finally Jack you noted the equivalent focal distance advantage of using FF glass on an APC-S camera. The other advantage is you aren't using the edges of the glass where we often experience distortion and CA, particularly when shooting wide open (even on some L-lenses).