Before I start here, I just want to make clear that you can field whatever you think is cool--this game is supposed to be fun, after all. However, if we're going to have a discussion about what is effective, that discussion is frequently going to tell you things you'd rather not hear.
I did say better than 'almost' anything. I didn't say absolutely better than everything.
I would contend that they do worse than most things, especially when taking into account what it costs to field.
For example the Manticore blows everything out of the water. I'd dare say it could even outperform that 7 lascannon example.
For the cost of a Manticore you could get 5 lascannons, almost 6 (or a solid 6.5 of them if you don't use heavy weapons squads). Just eyeballing the probabilities there, you're looking at a comparable damage output, overall. It is worth noting, however, that the Manticore can only shoot 4 times.
This doesn't mean the Demolisher isn't still a solid choice for a russ which is the main comparison here and in the chart, comparing the tanks to eachother and their potential damage output.
Even against other Russ chassis, I'd rate the Demolisher about middle of the pack. The Punisher is obviously the most useful, followed by the Executioner and then the Standard Russ. Then the Demolisher, Exterminator, Eradicator, and then the Vanquisher (which is cartoonishly awful). Overall I'd consider those last three to be so bad as to be essentially a waste of points. The Demolisher just clears that hurdle into "if you really want one" territory.
The deathstrike missile launcher for example will on average do about 5-6 guaranteed wounds just once per battle. This may seem like a terrible idea since it's once per battle but the fact that you can pretty much guarantee that many wounds on ANY target should be enough to at least consider it.
The Deathstrike missile launcher is, and always has been, a terrible idea. If you could shoot it whenever you wanted, then fine. Since you cannot do this and have to pay 40 points (!) more for it than you do for a Manticore, I cannot imagine why anybody would field it besides just for fun. 5-6 wounds is a laughably small damage output for *an entire game* from one unit. Just about every single other unit in the codex can reasonably expect to top this number (point for point) across 5-6 turns of play. Even the Demolisher is vastly superior.
The Earthshaker which you mentioned is GOOD but the Storm Eagle Missiles are shown to be nearly twice as effective so should we then say 'never ever take a basilisk' ? point for point the Manticore is surely surperior no?
You are correct. It should be noted that I did not say the Basilisk is good, but only that it is *better* than the Demolisher or standard Leman Russ (which gives you an idea of how bad *they* are). It's only distinction is that it is relatively cheap and, unlike the Manticore, can fire all game. Even still you're probably better off with Manticores, Heavy Weapon Squads, and massed infantry.
Anyway I think for the most part each tank has it's place, I own a demolisher and i intend to use it along with the punisher and once in a while the executioner which really isn't as good unless overcharging and killing itself.
Look, if you really like the Demolisher, by all means field it. But if you're asking whether it's any good or not, unfortunately the answer is "not really." I'm with you--I think the Demolisher is cool--but it got kicked in the teeth pretty damned hard this edition.
As for the Executioner, it has several things going for it over a Demolisher. First, it can take plasma cannon sponsons and still cost only 7-8 points more than a Demolisher without sponsons. Second, it has better range and the strength on its guns (especially overcharged) are basically just as good as the demolisher against any target with T7 or less.
The overheat thing is a problem, but a problem that can be at least partially mitigated by either tank orders or the expenditure of Command Points. I believe the payoff outweighs the risks in many instances, but I would say that the Executioner isn't all that great unless you make it a Tank Commander for the boosted BS (but then it can't get the benefit of its own orders, which is too bad).
Hopefully the codex will rebalance and give russes a break Large Blasts did indeed get the short end of the stick. But even worst are anything that had a Massive or Apocalyptic Blast...my Ork Kill Tank's bursta cannon is absolute trash now after winning me many games in 7th.
I think the blast thing can be easily settled by just giving large blasts 2d6 shots (or even just a set number of shots). Of course, everybody who plays against guard would howl about this, but in the end it would work out to the weapons doing *about* as much damage as they did in 7th edition and earlier.