The problem is that, for all his raw power, Bowser needs a read to actually get anything accomplished, in a metagame that does a fantastic job of not giving him one.
I don't want to derail this discussion, though, and it's been talked about at length here:
http://smashboards.com/threads/where-does-bowser-currently-stack-up-in-the-metagame.420987/
I'm just not afraid of overpowering Bowser so long DK's tools are very strong and he still hasn't really climbed into A-tier (though I absolutely want him to).
I mean, DK has...
-A spammable B-air with next to no landing lag
-A shield breaking option (that is nowhere near as punishable as Bowser's bomb)
-Amazing aerial mobility, allowing him to leap in quickly and catch enemies off guard (especially with that Bair which is also a godsend for ledgeguarding), which also allows him to drift in and out of range of attacks at the ledge for better recovery
-A cargo throw that allows him to carry someone off the side of the stage and then either attempt to stage spike them or throw them off the side of the screen while being much closer to it
-The ding-dong, or any number of obscenely damaging u-throw followups (he has a 67% combo on Ike that M2K found!)
-A chargeable KO option with super armor on it
-A great poke with Dtilt
-4 moves that spike, none of which require DK to SD in the process
...And for all of this, I still don't even see M2K regularly taking tournaments with him.
As someone mentioned in one of the Bowser threads, there's no point in feeling bad about the potential for DK, Bowser or any HWs to be OP.
People played Diddy without remorse, and they still play Sheik, ZSS, and Rosaluma without even a hint of feeling bad for playing characters that should probably all be toned down a great deal.
I hope they keep adding ding-dongs, combo throws, and whatever else they can to HWs to make them viable at competitive level without being OP in casual play.
One day, I hope this game is so well balanced that Evo's grand finals could come down to a battle of DK vs. D3 and no one would find it unusual (though, I'm not holding my breath for that...).
I think the real problem is that there's only so much a game developer can do with an archetype like heavy characters in a game that's primarily designed for a casual audience who will treat it like a party game.
In essence, we've ALWAYS been trying to fit a square peg through a round hole when it comes to turning SSB into a competitive game, but again, I feel like Smash 4 has tried the hardest of any of the games yet (though that doesn't really say much).
I'm not a huge fan of throw followups as a means of balancing, but at the very least, a throw followup is something a competitive player will know how to use when a casual player won't. It's essentially an "advanced tech" hiding in plain sight.
But I can definitely see where you're coming from about heavies and design. The real trouble is, IMO, not even the heavies themselves but the fact that he best characters in the game completely negate their advantages.
Heavies are designed with the idea that a character will survive longer and hit harder, dealing more damage with each hit. This looks great on paper, but the key problem is that SSB4 has COMBOS in it, and if a heavyweight is more susceptible to combos, then it tends to negate a huge chunk of the justification for their moves being generally slower. That's one strike against heavies.
The second (and arguably worse) strike is that the best
defense in SSB4 is typically a good
offense: characters that can throw out quick attacks in rapid succession can make it very, VERY difficult to counterattack them (ie better frame data). Again, this is where SSB4 being a casual game at heart hinders the competitive side because things like super armor could go a long way toward making heavies balanced in competitive but would make them godly in casual, so they clearly can't do THAT, either...
TBH, though, I think the biggest buff to heavies would be a nerf to some of the higher tiers that give them (and most other characters) so much trouble. Characters like Sheik and ZSS are as strong as they are because they can put out a such a powerful offense while having to risk very little. Even just adding a few frames of endlag to some of their moves, especially the highly powerful ones (and they did this for Diddy's Uair), might make the spread between the tiers much smaller and make heavies HUGELY more viable.