Ah, my favourite kind of post to ruthlessly disassemble.
can't move at practical speeds
A Superheavy's job isn't to be fast, and I could probably dig up explicit book quotes of Baneblades or whatever moving faster than they have any right to if I had the time.
also is giant target
Which could well be the point, it draws fire onto its heavier armour and thus away from more vulnerable targets. This concept doesn't work IRL because armour is becoming increasingly less relevant in the face of modern offensive technology, but 40K has plenty of super armour materials which are better than anything that exists in real life.
and one shot in the tracks would destroy it provided it's a explosive
You do realise that this applies to any tank, or any vehicle really? And here we see that you haven't done your homework, armour-piercing rounds or shaped charges are actually more effective than conventional high explosive for attacking an armoured target.
also 200mm isn't even that good and that's for a baneblade for crying out loud
200mm of materials better than anything we have today. Unless you lowball really hard and deliberately cherry pick the worst showings for everything, at which point it's not really Warhammer anymore - not just thematically but in the sense that the setting stops working as a coherent entity.
in fact aeronautical weapons make super heavies even more obselete as do artillery
Once again you show the half-hearted level of research you've done. Every army in 40K practices combined arms, especially the Astra Militarum who make the most use of superheavies. A Baneblade or suchlike being deployed by any competent commander would have anti-air weapons and counterbattery fire to support it.
a baneblade woudnt work any better in 40k than 2017
"This idea doesn't work in real life, thus it totally also wouldn't work in a setting which repeatedly demonstrates vastly more advanced technology than anything that could possibly exist in a realistic world, much of which actually biases toward the justification of such things."
just I giant slow target that is prone to breaking down not to mention refueling that behemoth
Well, it's a good thing the Imperium canonically uses fuel with an energy density millions of times higher than ordinary gasoline.
and what on earth could carry that off orbit onto battlefields
Much larger and heavier things are canonically deployed from space all the time in 40K.
still even with void and airpower obtained artillery would be a huge threat
If you're using artillery to attack tanks you're either using the artillery wrong or desperately short of proper anti-tank weapons.
not to mention lascannon platoons melter guns etc.
Back in the 1950s people were saying that ATGMs would eliminate tanks from the battlefield entirely, yet sixty years on modern militaries still find non-niche use for MBTs despite ATGMs being far better than they were then. The existence of anti-armour weaponry does not in and of itself make armoured vehicles obsolete.
Also baneblades arnt worth the rescources considering 1 baneblade worth of metal and other materials might as well be 8 lemon Russ 2 valkyries and 12 chimeras with a full complement of full blown imperial guardsmen
I'd like to see the actual calculations you did to reach these conclusions about the proportions of resource expenditure, and the in-universe sources you have to back them up. Especially I'd like to see where it's stated that the same resources are being used to make Baneblades and Valkyries, given that automotive and aeronautical engineering are separate fields and the products thereof require different materials to do different things.
I won't argue that there is plenty to criticise about the Baneblade, it's a silly design like most Astra Militarum tanks. But the concept of superheavies in and of itself does make sense within the setting - at the very least it's a damn sight less ridiculous than the Titan-class war machines which actually do make no sense. I'd recommend you do a bit more homework on both 40K lore and military doctrine in general.