I thought this was some kind of joke... until I realized they skipped v3. But still, v5 already? Or did I just get used to Bootstrap 4 being in Alpha for years?
This is common for end-user apps but feels very weird for developer libraries and frameworks.
We are used to X.Y.Z releases where X are "major rewrite", Y are significant updates, and Z are bugfixes.
That said, Angular's new scheme could be technically correct strict "semantic versioning". In semver X.Y.Z, Z releases are supposed to be bugfixes with no feature additions, Y releases add features but with perfect backwards compatibility, and X releases introduce breaking changes. A backwards incompatible change, no matter how small and easy to adjust for, should be a major version bump according to semver.
I work in a large corporation and they're still planning their transition to 1.5 and then to 2. I'm keeping up by building stuff in my off time, but trying to keep a fortune 200 corporation on a six month upgrade track? Not happening.
I've already heard rumblings from upper management of going back to some open source Java frameworks like Spring MVC, Vaadin or gulp Struts because of how fast they are releasing major versions and the time and cost of keeping up.
My fear is Angular will be regulated to startups, medium sized businesses or small autonomous business units within larger companies. When faced with a six month upgrade schedule, many larger organizations I fear are going to balk and just drop it in favor of something they are already comfortable with. . ergo my company wanting to go back to Java since 95% of everything built here is done with Java.
Angular 1 -> 2 is an actual "major release". "Rewrite all your apps from scratch" major.
But starting from Angular 4, the "major version number changes" doesn't mean a major effort to update. There's a decent chance that 4 -> 5 -> 6 -> 7 etc will work without any code changes. But as long as there is a backwards-incompatible change, no matter how small and in how obscure submodule, it requires bumping the version number according to semver.
Most enterprise users will probably skip multiple versions between upgrades, just as they skip Ubuntu releases and Django releases and Rails releases and so on.
Joking aside, apart from semver another reason for the weird versioning in Angular is that it's a rather large collection of modules, especially if you include things like RxJS (which technically aren't exclusively part of Angular but affect APIs due to their widespread use). So it's fairly easy to get a semver major version bump because of an implementation detail you likely wouldn't care about but that breaks backwards compatibility with some older library in some way.