If that was the case that's all we would see on NA1 which isn't true. It's mostly medium armor. I don't even wear full plate on NA1 anymore.
There's no upkeep on the duel server. Despite the fact that plenty of people are sitting on piles of gold, never underestimate a sperg's need for more gold. Not to mention there is no real reason to tryhard on NA1, while a tournament has both prize and pride incentives to win. People typically find it more enticing to be able to zip around the battlefield rather than tank a few extra swings, so they wear lighter stuff to have more fun. Also, mobility is probably more important when you have to deal with ranged/cav on large open maps with teams of 30 players per side, not so in a 2v2 melee-only tourney. Lastly, not everyone has a fully loomed set of plate armor. For instance, I've got a loomed set that equates to ~25k gold... if there were no gold limit, I'd only be hindering myself by using such comparably-lightweight armor. If there were a gold limit of 30k, well then people wouldn't have armor that much heavier than my own, so it'd be close enough.
Let me ask you this: is having no money cap at all changing how you are planning on playing?
Dunno if I'm planning on playing at all, but if I were, in short, yes it would. Even if I didn't suit up in the heaviest armor I could muster, I'd still be far more inclined to bring a blunt/pierce weapon over a cut one as the likelihood of there being only heavy armor drastically increases.