Its more of a matter of smaller destruction, nukes did evolve into smaller wields, such as the AIR-2 Genie that was a anti-bomber missile,
Genie was a terrible idea given that it was an unguided rocket and not a missile from that terrible era in the 50s when guns were ineffective and missiles were still embryonic.
https://en.wikipedia.org/wiki/Battle_of_Palmdale
I kinda like handwave things because I recall when Vietnam started there was the idea missiles had replaced guns entirely just to be proven completely wrong
Except that they had replaced guns entirely. The issue was the ROEs in Vietnam made by McNamara and the DOD prevented combat beyond visual range that completely negated the strengths of, and reasons for, missiles like the Sparrow.
In a similar way North Vietnamese integrated air defence had spelled the end of aircraft operating below 10,000 long before MANPADs did. The only reason terrain hugging and aircraft like the A-10 (which wasn't built to fight in Europe, it was made to replace the A-1 in the light ground support role and was obsolete before it entered service) were able to keep going was that circumstances after the COld War allowed them some room to be of use even though terrain hugging aircraft and the A-10 suffered horrible losses in relation to other aircraft in the Gulf War and the main reason the latter was sent Scud hunting was to get the aircraft out of harms way and still be of use.
mechs in BattleTech do in the sense they ended up as such scaled up Infantry
I don't see that in a Mechs role, at all. What Mechs do and what infantry do have little in relation to one another, and if that is the mindset than it's foolish for creators. Infantry will be replaced by other infantry, either exoskeletons in BT, or by robots, which is why the role of infantry hasn't changed despite the dramatic changes in warfare in the last century, nothing can replace boots on the ground but boots on the gorund.
The nearest thing Mechs are are like old and armoured cavalry with their increased mobility and firepower enabling them to act as flankers and mobile support for BT-suitable infantry that could vary from high exoskeletons and battle armour to simple uniformed infantry like we have today that are backed up by conventional vehicles and equipment as we know them, aircraft, tanks, artillry, infantry support weapons like mortars.
With that said, I could also see Mechs acting like the fulcrum of warfare like heavy cavalry was during the Middle Ages, where most other parts of an army showed up giving half-hearted resistance as they watched the knights fight it out that would then rout if theirs lost whether or not they still had a chance to win the battle.
I'm not familiar with much BT fluff beyond randomly browsing Sarna while fighting insomnia, but has there even been battles like Agincourt or Sempach where defenders lacking mechs defeated a mech force using prepared or environmental defences to eliminate the mechs advantages and exploit their weaknesses? (Like fighting in mountains or rough, rocky terrain)?