CentOS keeps the same major/minor versions of (at least) any significant packages through a release’s life cycle, which does result in some pretty old versions of software. They do backport security fixes for those packages, though, so you aren’t left with vulnerable software. And for packages where you might really want a newer version (like PHP or MariaDB), Software Collections let you install newer versions alongside the default packages.
I’d be concerned that using third-party repos for core packages would result in a less-stable (and/or less-secure) core system (particularly for PHP, whose devs appear to treat backward compatibility as–at most–an afterthrought), though I do understand wanting to be able to use current software. What problem does your suggestion solve that Software Collections doesn’t?