Direct HEAT is indeed, highly ineffective against modern tanks. I don't recall ever suggesting it's use. A tandem HEAT/EFP top-attack ATGM will incapacitate just about every tank in use. Basically every modern ATGM operates on that principle. This is partly the reason I brought up range, which I should probably rephrase. The main reason to mount ATGMs on a tank is that they have top-attack capability, which is pretty much the most effective method of destroying any tank. They have better penetrating ability (especially against ERA) and a much higher chance of catastrophic kill than APFSDS. The extra range is useful in an admittedly small number of circumstances, but not the main reason to utilize them. I didn't mean to give that impression, I apologize if I did.
Also a reason not to use ATGM carriers is you're putting all your eggs in one basket. I'm pretty sure this is why cannon-fired ATGMs were used in the first place. If you have an armored force, say for the purposes of this, 4 tanks and one ATGM carrier with 10 missiles, and you lose your ATGM carrier, the group immediately loses 100% of it's ATGM capability. If you have 5 tanks with 2 missiles each you have pretty much the same ATGM-firing capability.
Also if you don't want to spend time vulnerable while guiding the ATGM, then just don't. Semi-active system on the turret can track a target as it would for it's main cannon. A modern IIR or ARH guidance system is fire-and-forget. The impact on the tanks normal operation should probably be less than a cannon-fired ATGM, since the tank doesn't have to load a shell after firing it's ATGM(s). Most full size ATGMs are 25-50 kg. If we allow for even 300 kg of electronics, mounting, etc. and 4 missiles, that's at most 500 kg. Not even 1% for an Abrams M1A2. Pretty much every tank I know of has some mechanism to allow soldiers to mount with a fixed turret, whose body and equipment weight exceed that by a fair amount.
The costs of implementation and use shouldn't be much different than that of cannon-launched ATGMs, other than if you're mounting more advanced or larger ATGMs that aren't compatible with the cannon. Any ATGM could be mounted, some of them cheap, some of them expensive. I proposed this an alternative for countries like the US that don't currently use cannon-fired ATGMs, but have developed existing, effective ATGMs that are used on other platforms. Doing so would require either importing from another country or spending money on R&D. While researching it I concluded it does seem superior to them in a few ways, and while they are more expensive, I would rather have a more combat effective tank than a cheaper tank. How resources are spent is a matter of military doctrine which is going to vary from country to country. You don't have to be so quick to use absolutes.
