>You can quickly lose a trademark if you don't actively defend it (weird, considering patents and copyrights) so companies quickly get over zealous. //

It's oft repeated but not true AFAICT. Genericisation can be a problem, but another company using something as a trademark for a distinct type of good/service won't make your mark more generic. You lose a registered trademark by not paying your renewal fee.

If you wanted to codify someone else's use then you could grant them a license, you don't have to sue them or start infringement proceedings or whatever. You're free to ignore one infringement and pursue another.

This is like saying you have to prosecute every person who steps on your land or you lose the right to prosecute trespassers; if you allow a right of way to be established then yes that's probably not going to help you. But you're allowed to let someone in to your garden and you're allowed not to sue people who make incidental incursions.

Trade mark law does not forbid trade marks on common language words and phrases. It generally prevents people from registering 'descriptive' or 'non-distinctive' words/phrases as trade marks. "Windows" is descriptive for glazing supplies so wouldn't be registrable. "Windows" for software is distinctive and there is no public interest in it remaining free for anyone to use, unlike the case with the glazing example.