Act bosses never took 5-10min unless you're talking about people who experience those fights for the first time. Actbosses also aren't endgame, so having to slog through multiple 5minute fights would be rather boring during the levelling process. Neither Maelstrom of Chaos during Beta nor the first incarnation of the map-system had fights that took nearly as long as you describe, unless you're talking about shitbuilds. Those never have been able to kill bosses fast, and they don't do it today either.
Healing Flasks also never had to be used smartly, you usually just rolled 3 seethings with a bubbling here and there and you were good to go. Back then the only flasks available all needed 10 charges to recharge once and every single non-map Boss already had a steady supply of minions to kill for those. Flasks never were an issue, especially with builds being able to permastun/freeze bosses anyway. Is CI+VP+GR stupidly strong? Certainly. Is it the only way to get through the hardest of contents? No. Being able to leech through stuff like Shaper Beam, Mino Caveins or other stuff has a huge gear-dependancy that comes with it, something that Life doesn't have to worry about. Get 6k+ Effective Health on HC, get a second defense layer through evasion/block/dodge, get 2 seethings and you're good to go up until and including Shaper.
High enough leech always has been the strongest defensive option unless you can perma-control enemies. Smiters did it in Uber Tristram, Freeze-Mine Prolif/Ground Slammers did it a couple of years back in PoE vs bosses, and you almost always had a Freezepulser for non-bosses in your party aswell. Dying back then was harder than it is now because stuff like this got nerfed. And so will double-dipping/perma-leech - eventually.
I would also like to remind you that leech, and by extension CI, were in a pretty rough spot after the leech changes about a year and a half ago. It's only been able to revive thanks to stuff like Vinktars providing enormous amounts of leech, ToH/Basalts providing high mitigation even with 0 armor (remember when every CI player wanted a T1 armor roll on their belt for just a couple of percent of mitigation), and CoH/Blasphemy+Warlords Mark providing both leech aswell as additional mitigation that you previously lacked. I remember when I had to stand still and enduring cry every 10seconds for 4 charges. We basically got automated things that used to make CI unattractive for the majority of people. Back then people complained about life being the "only" way to play, which then got most life-nodes nerfed and CI received the 3 nodes behind it).
The game and meta constantly change, always have, but it has indeed gotten a bit stale. GGG used to be rather quick with nerfing stuff, but people constantly whining about getting nerfed in the middle of a league managed to put them on the defensive which is why we only get major change to mechanics once every 6-12 months now, when a new expansion hits. It is also a lot harder to balance properly and not just bandaid-fix stuff once your game has gotten to the size PoE has grown to today, which is probably why nerfing/buffing certain skills is no longer a top-priority. And as many people will tell you, the game is far from unplayable unless you conform to the meta (and therefore validate it).
The meta is exactly that: Something that develops over time due to the way the majority of the community views the core-mechanics of the game. The more complex the game, the more options you have. Can the game nudge you in one direction? Sure, look at LoL. Can multiple completely different metas evolve without the game changing at all? Sure, look at DotA. People only perceive what they see in front of them, and since the majority of what is visible is people crying about "the clearspeed meta" it is nowhere close to being as bad as the vocal majority might make you believe. Once you realize this fact and simply don't conform to it, you will realize just how unimportant a lot of the issues are. That's not to say that some of the criticism isn't valid or at least holds a grain of truth, but having played HC since Nemesis I can tell you that none of the complaints really mattered in the grand scheme of things, ever. You can still have fun in red-tier maps even if you don't breeze through them in a matter of seconds, you can still enjoy the game without perma-leeching through shaper beams and most importantly: You can still play the game in whatever way you want. As long as you actually know the mechanics and core of the game you can do any content that isn't inherently supposed to withstand your build (like being unable to kill Uber Atziri with a build that solely relies on ignite).
The whole thing about blue maps providing close to the rewards a hard rare map does is utter bullshit btw. If you cannot handle the IIQ-improving affixes on a map, your statement holds true. Do people run high-tier maps and skip bosses when its -max/added ele or shitty curses that fuck you over? Yes. Is it about 3times as profitable as running a perfectly rolled blue one? Yes.
Just as an anecdote: I have been playing AW summoners for the past 4 leagues. It has become a "meta" build due to severed in sleep/united in dream, but I can tell you that the build didn't really feel different at all. What was decried as a "shit skill" has always been used by people who were less intent on bitching, and instead enjoying the game. And it always was able to clear the highest and toughest of content, even without all the "gamebreaking" uniques. There certainly are skills that need a buff to become viable, but there are way more options to play the game than most threads that discuss the clearspeed meta/double dipping may want you to believe, and those options are oftentimes only slightly worse than those top-tier skills, and in some occasions are actually better, people just don't believe it until someone shows them.