There is a ton of software that will never be ported to ARM.
Newsflash: Apple have already announced that - ARM or not - the next version of MacOS won't run 32 bit apps. Like it or not, a cull is coming, and quite a lot of those apps that aren't being actively developed are already dead software walking. Meanwhile, for a huge tranche of software written in C/ObjC/Swift and using Apple frameworks for graphics/vector processing etc. "porting to ARM" will consist of ticking the ARM box in Xcode, re-compiling and doing a bit of testing (oh, wait, its 2019 so forget the testing...) - *nix stuff (Apache, python etc.) is mostly platform-independent C/C++ (as per the Unix tradition) plus they are already supported on both ARM Linux/BSD and x86 MacOS so "porting" shouldn't be a big deal c.f. the laundry list of obscure platforms that they
already support.
Will it also use a browser that can’t run Javascript or just not have a browser?
Newsflash 2: Safari for iOS runs Javascript. Chrome for Android runs Javascript. The Raspberry Pi runs Javascript. Javascript runs fine on ARM.
As others stated, why would a developer purchase a Mac if this rumor turns out to be true?
- developing for MacOS
- developing for iOS (already a big seller and developing on an ARM will make testing/debugging easier)
- developing for Android (ditto the above)
- developing for Unix/Web/Scripting (Apache, Ngenix, node.js, mysql, mongodb, python etc. all work on ARM and MacOS/ARM 'ports' are available).
...in fact, pretty much everything short of developing for Windows (which is hardly the strongest reason for developing on a Mac) or testing Mac apps on an Intel Mac* (no reason that you won't be able to
build them - and if a developer doesn't
have facilities for testing on different hardware configurations then they're not a serious caller).
Bottom line is, the days of
application developers needing to worry about processor architecture are rapidly receding.
There's no doubt that not being able to dual-boot/virtualise x86 Windows and Linux will be a drawback, at least in the short term, and, yes, Apple
could mess it up by trying to force the transition by abandoning Intel product lines too quickly. 'Locking down' Mac OS and Mac hardware too much would also be a problem (e.g. so the Linux community weren't able to get ARM Linux running on a Mac). But let's not assume the glass is half empty until we at least know that there
is a glass!
* And that's assuming that Intel/AMD will block Apple from doing x86/amd64 emulation (which is a solved problem apart from legal issues) - I wouldn't bet against Apple's legal team (esp. as Microsoft is already doing x86 emulation so they'll be Apple's frenemy on that) and Intel and AMD would probably quite like to keep selling their other products in a post-x86 world. Its not as if Apple is the only company sniffing around ARM as an Intel replacement.