> You can probably get 90-95% of the benefit of git using mercurial with a fraction of the effort in learning.
You absolutely cannot.
Even if we pretend that Mercurial is simply outright better than Git, there's a lot of value in learning Git specifically, as it's what most development teams use. Employers value proficiency in Git. If you mention your Mercurial proficiency in an interview, it's likely to be scored up as cute but irrelevant.
If you always work alone, sure, Mercurial might work great for you, but the real value of these version-control systems is in enabling teams to work effectively. Most teams these days use Git, so you need to know Git.
(Of course, if your team does use Mercurial, you'd better become proficient in Mercurial.)
> Life is too short to spend learning the internals of one version control system.
I agree that learning the intimate internals of Git's codebase isn't something that's likely to pay off in the day job, but short of that, Git's 'useful skill-ceiling' is pretty high.
> Don't waste your non-work time on learning computing related stuff and instead spend time on social connections and physical fitness first.
Depends on your working situation. If your work offers no opportunity to learn new skills, and you don't want your CV to get stale, you have little choice.