Microchip has been sharing their source code for many years.
They are not out of business.
They're in the business of selling proprietary hardware, not software.
Thus, whatever they do with their software is totally irrelevant.
If a '51 manufacturer shared anything it would be used as well with a competitors product. I quote (forgot the source) "I develop on the cheap SILabs developmnt platform and then migrate to other chips". Would "releasing the source code" not make that situation far worse ???

For better or worse, they have a larger market than 8052 vendors.
Not true.
what is true
"For better or worse, they have a larger market than any individual 8052 vendor, while there are more '51 derivative chips sold than PIGs"

Erik