Keywords on their own don't do much. They only were interesting because they keyed off powers - and 5e at launch had a commitment to not having that level of customisation.
I disagree. 5e already has things like versatile, finesse, and reach. You could easily add in brutal N, high crit, defensive, the various "loading" properties (e.g. "load bonus" instead of "load minor"). I do of course believe that the weapon
groups also added something, but that's a separate issue that would, as you say, require actually making such rules matter to the baseline classes. But even there, 5e has the ghost of such options, with both fighting styles and feats that specifically benefit from certain options (e.g. great weapon fighting, Great Weapon Master, Polearm Master, Shield Master, in theory Sharpshooter and/or Tavern Brawler). It
easily could've been implemented in a fairly natural way, one that would've allowed significantly deeper options without adding much more
complexity, given, y'know, we already have feats etc. that
effectively trigger based on what group a weapon belongs to.