Why don't Leica make SLR lenses for other makes now the R System's gone?
Their production capacity is fully committed just to support their own systems. They have a months-long backlog of orders for M lenses despite ever-higher prices, and they have plans for further development of the M and S systems and possibly for a future mirrorless digital system.
Why are there seemingly so few used late model R lenses out there for sale? I've been looking and looking for a 2.8 90mm for my dad's R9.
A dealer I often buy from - www.vintage.visuals.com - has a selection (which changes constantly) of used Leica R lenses. No 90mm at present, but you can check in from time to time. He is based in Calgary so you don't have to deal with customs/exchange rates/extortionate shipping, etc. I have dealt with Vintage for over a decade and have the prices there to be reasonable and the grading to be quite consistent.