The suggestion by the parent of my comment was to replace the system Make with GNU Make because others are using GNU Make specific features.
On top of that you want to rewrite all of the Makefiles for other implementations of Make, that exist on systems where the implementation of Make that is included with the system is not GNU Make, so that the system still works after you've switched out the implementation of Make that came with the system with GNU Make.
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Why?
Why should everyone else do a bunch of extra work just so that the projects using GNU Make can keep using GNU Make specific features, when literally all those people using those GNU Make features had to do was to rename their Makefile to GNUmakefile and it would keep working for them when they invoke GNU Make just the same way they did before and would not cause confusion for others?
If you are talking about a bunch of BSD userland, probably the only place that uses BSD extensions and other gnu incompatible non-posix stuff, I think it's pretty easy to hardwire it to only build with BSD make variants. Who even builds it manually, and how often? The OPs comment was about software in general, and for software in general gnu make is more optimal as a default in 2019, that is unless you have to be standards-compliant.
On top of that you want to rewrite all of the Makefiles for other implementations of Make, that exist on systems where the implementation of Make that is included with the system is not GNU Make, so that the system still works after you've switched out the implementation of Make that came with the system with GNU Make.
...
Why?
Why should everyone else do a bunch of extra work just so that the projects using GNU Make can keep using GNU Make specific features, when literally all those people using those GNU Make features had to do was to rename their Makefile to GNUmakefile and it would keep working for them when they invoke GNU Make just the same way they did before and would not cause confusion for others?