FormerlyHemlock
Hero
How do you "slam them prone" without some significant investment? Knocking people prone isn't something you can do without a feat, a special effect (e.g. wild shape), or a spell, as far as I was aware. Even with that taken care of, both of those things are contested vs. the target's choice of Dex(Acro) or Str(Athl)--unless every character is proficient with Athletics and pumping Str, they're not going to be able to keep up with monster stats (since most monsters have a good score in *at least* one of those once you get to mid/high level).
You don't need a feat or anything. It's built into every character. Review PHB 195. DMG has additional variants that make Disarming built into every character too.
Monsters are not generally proficient in Athletics (Stone Giants and Fire Giants are an exception) so even a Str 12 Dex 20 archer will be halfway decent at grappling if he wants to (+5 to Athletics at 8th level, and his two attacks are basically equivalent to advantage) and e.g. a Str 16 Cha 20 Paladin will be quite good. I speak from experience. I have a Str 16 Cha 20 Paladin who was in no way built for grappling, but sort of stumbled into it as a good strategy maybe 20% of the time (fights against one to three not-Huge creatures when ranged support PCs are present) and despite having only +7 to Athletics and two attacks, it works pretty reliably. You don't always get to grapple them and slam them prone in the same turn, but you can do one or the other about 75% of the time, which means about a 75% chance of removing one enemy from the fight. That's better than most spells can do, and it's completely free except for the HP you lose, which are easy to heal.