The real improvements all came from the hard work of the developers who were around during the 7.* releases who did excellent work.
Most importantly at that time we had Nikita Popov who was incredibly helpful.
Hack/HHVM definitely gave it a nice kick of motivation though.
If I think about swift I think about ios apps (I know it can be more today, but their marketing for this language wasn't good).
So apple never wanted big adoption outside of their devices.
Swift isn’t gaining much adoption because Apple aren’t putting much effort into promoting its use outside of the Apple ecosystem. And why would they when they don’t care about non-Apple stuff
C has had multiple implementations from multiple implementors for decades. Java wasn't really tied into Sun's ecosystem, and had an enormous marketing blitz. TypeScript was even less of a Microsoft "company language" - and I think it's interesting that Dart felt a lot more tied into everything Google was doing, but ultimately lost out.