I'm mostly a support player and I side with OP on this one. I've been feeling this way for a while.
The power curve of Dota is becoming more and more flat, for everyone. IceFrog seems to be driven by the goal that everyone has to be able to have closer and closer to equal impact (and, by assumption, equal enjoyment) at all points in the game. I personally think this is less interesting both tactically and enjoyment-wise, but I seem to be in the minority.
Maybe it's my bracket (~4k), but lately Dota has been just a straight slug-fest for map dominance from almost minute 0 to the end. Well, more accurately there's sometimes like a couple minutes of "laning phase," then after that it's just constant fights beacuse of how much gold/exp value is gained from kills and assists.
There is almost never any "downtime" in the game anymore. The game seems more driven than ever by cooldowns and death/buyback timers because the game is just a nearly-unbroken chain of fights, and frankly there's less tactical control as a result. It becomes more reactionary for both teams to have fighting as far and away the largest determinant of exp/gold and overall advantage. You have to respond to engagements or risk falling behind, and it has this all-encompassing effect on fights from the midgame onwards.
I liked Dota more when engagements were more measured and taken with more tradeoffs. Fights were not just a revolving door of all the heroes on both teams. Split pushing was a real threat. Giving the enemy carry too much space/time to farm was a real threat.
In the current meta, pushing ends up as an afterthought after a clash, and objectives are picked up with the advantage created once the teamfight swings get large enough in one direction or the other (or after waiting out successive Roshans when high ground proves difficult to breach). The power of kill/assist gold advantage honestly trumps the risk of letting most enemy carries farm in peace.
Maybe I'm in the minority, but as a support player, I actually liked it when my carry got overwhelmingly fat because of sacrifices I made - the vision I provided, the stacks I made for them, the deaths I took in their place. I'd rather have a fat carry and myself be underfarmed than a more equal gold distribution and more equivalent fight impact late in the game. A large part of the enjoyment of being a support player for me was setting up my carries to succeed and then having that pay off in the lategame, not necessarily having X items of my own and being able to have a surprisingly large effect on fights with all of the items now at my disposal.
Yes, all 5 players on each team should have a significant impact on the outcome of the game, but for me, playing support meant that a significant part of my contribution was actually in how strong our carry became due to the advantages I helped create for them. Apparently I'm in the minority, and most support players these days are in it for the blood and that juicy kill/assist gold.
I find it deeply ironic that the top-level comment about self-declared "self-sacrificing support players" is that they "don't play for the scoreboard...[and] play to win through tactics and plays," and yet somehow "the only reward is making plays to earn kills/assists and thus gold." It smacks of the "gold is everything" mentality that is purported to be the ailment of "1v5 carry players" to put such an emphasis on both the magnitude of the self-sacrifice of playing support and the value of kill/assist gold. I think it says something about the state of the game when the overwhelming priority of supports is kill/assist gold.
I had more to say, but I'm cutting it down for length. Honestly I think /u/seppuku4kstew is hitting the nail on the head: the crux of the problem is that the punishment for fighting all the time as 5 is too weak at the moment (or conversely the benefit is too high), so there's no real reason not to have this as the dominant strategy every single game.