I think you are correct mathematically, and perhaps in terms of potion usage, but it is important for a pokemon to have enough HP to survive a battle. Quadrupling the effective HP of a Snorlax will have less influence on its practical chance of winning a battle than quadrupling the effective HP of an Alakazam.
Why do people think dodging help glass cannons more?
Dodging effectively quadruples your HP at the cost of a few seconds of your DPS.
Those that gain the most, then would be those with the most effective HP while those that pay the greatest cost is those with the highest DPS
Answers
Exactly. For many players nowadays the problem really isn't taking down the gyms (even more so with any low level gym). What you want instead is just to be as time efficient as possible and that is why Pokémon with high attack stat are often preferred to a slower attacker (like Snorlax).
Alakazam will kill faster while dodging than a Snorlax who's not dodging.
Sure, it's more challenging and kind of satisfaction to attack a Lvl 10 gym with six mons and actually using only five of them, but when you're standing in the freezing cold because you want your 10 coins it's tempting to f* the full clearance bonus and instead of kicking two fat Snorlaxes in the guts for seemingly endless 40 seconds just get the same 1,000 points much faster by beating the CP 700 Hypno at the bottom.
Excluding the impact of the difference between quick moves and charge moves, the gain in HP is a percentage and the loss in DPS is a percentage. Both being constant percentages. Thus, dodging helps everyone EQUALLY, by a constant percentage.
This of course is thrown off by differences in Quick moves and charge moves. i.e. a 50 energy charge move with massive DPS for its cast will benefit more from dodging compared to a similar 100 energy charge move because you won't waste energy on it. An extremely higher damage charge move wastes less energy than a slightly more powerful charge move because you're sacrificing mostly quick attacks rather than charge attacks when dodging.
As to why Glass Cannons benefit more from dodging? Simply put - time is important. If I can kill 3 pokemon with a glass cannon in 20 seconds with dodging before swapping him out at 1HP, but can kill 4 pokemon with a sturdier attacker in 35 seconds - the latter does more work but the former is more efficient. Without dodging though, the former kills nothing while the latter gets a kill - so the latter is better. Hence dodging benefits glass cannons more.
I'm fine with everything you said except "Thus, dodging helps everyone EQUALLY, by a constant percentage"
A constant percentage does not an equal effect on each pokemon.
I'd say glass cannons make dodging more important, but would not say it benefits them more.
Also nor sure using a glass canon like Alakazam is going to save you that much more time then using a sturdier attacker like Vaporeon