We already had a situation with mobile anti nuke in sea on Reef which was removed for good reasons.
Mobile anti can be moved in a matter of seconds covering another area, can be cloaked and almost impossible to scout and destroy without destroying everything else in path to it. Such situation is ground for stalemates and porc festivals which are way more boring that nuke.
Proper and efficient usage of nuke requires - skill -., especially when you are in a situation where u are lacking metal.
Just going to describe what I do when managing my nuke under tight conditions.
1. Stockpile at high priority with metal reserve enabled.
2. Limit stockpiling to a single missile to allow scouting/anti disable.
3. Scout, depending on map it takes 5-10 swifts + air factory cost. Scouting alone works just for a short period of time as enemy may make new anti nukes and you have to re-invest in scouting. This obviously is not free and costs metal.
4. Now you need to disable/destroy previously scouted anti nukes. This again requires both time and metal, be it either silo or widow
Failure at steps 3-4 leads to a failed nuke attempt and puts you back a fortune(5k or so metal - shot, scouting cost, disable anti cost). Nuking area covered by 3-4+ antis can be a real challenge and it's pretty interesting both to execute and watch.
So as you can see nuke never comes alone(maybe only if it was rushed and enemy was dumb enough to not scout it - well they deserved to be nuked then).
Nuke = silo cost + missile cost + scouting cost + disable anti cost + time to do all of this before the scales is tipped in enemy favor and you lose the game.
Mobile anti completely ruins any interaction between nuke offense and defense which is bad.
You could try to make static anti cheaper but it may ruin the balance and make nuke useless(it's already kind of been nerfed directly and non directly couple of times - anti now protects in all nuke fly-by area instead of just zone where anti placed and nuke silo used to be cheaper before)
There is also a concern about map size - the bigger the map, the more efficient nuke is. So, say, on 12x12 map 3k cost anti nuke is insanely cheap as a single anti covers most of the map, but on 20x20 map 3k cost anti may be pretty expensive to cover all vital areas.
But again, this is in the current meta-game where lobsters like to sit and make porc cities instead of making a dynamic movable army which is way less susceptible to a nuke. Let's say all lobsters will expand and make mobile army in 20x20 map, - nuking front will be almost useless and back can be protected just by 2 anti nukes which is very hard to destroy(out of silo range, covered by 200m flea line, team has air to intercept bombers).