From the single or none fulfillment of broadband options in the US even in dense suburban areas, let alone rural areas, it is clear that there are places in dire need of similar government market regulation.
Portability of embedded-code source, standardized ISAs, and ability to turn around a new circuit board design in a week, and FPGAs all help make lack of second sources survivable.
I suspect second-sourcing must have been going on fairly early in the 6502's lifecycle though. Commodore OWNED MOS by the time the 6502 was a shipping, finished product, so they're obviously going to dogfood it on the PET/CBM/VIC series.
A 1977 design like the Apple II had a bit of plausible deniability-- they may not have known that Commodore was going to be a major player in the small-computer market when they did their design, and then they were stuck with the 6502 and derivatives for backwards compatibility for the rest of the series.
But then you have machines like the 1979-release Atari 400/800. By then it was obvious you're buying from a potential hostile competitor. There must have been enough confidence in their business practices or the availability of second-sourcing that they weren't afraid of being cut out of the market.
CPU-World shows a bunch of 6502s from different manufacturers with date codes in the 1980s, but I can't comment much more about if they were clones, or license-built second sources, or what.
Aside from the NES, I can't think of any major greenfield 6502 products much after 1981, but that's likely less about second sourcing and more about a deluge of cheap Z80s and 8088s making the 6502's low price less compelling.
The 6502 succeeded by being 1/10 the price, and was designed into products of companies that embraced extreme risk.