In the overall context of the occupation, the Antonine Wall doesn’t appear to have been very useful. Including the time taken to build it, it was manned for only around 20 years.
Most of Hadrians Wall is built on an outcrop of the Great Whin Sill which gives it additional defensive geology. How does it compare in terms of military effort compared with the Limes in Germany?
Although there are remains of a substantial amount of Roman infrastructure in Scotland, I reckon the Romans pretty quickly decided it wasn’t worth the effort. North of the Antonine Wall, the land isn’t very productive, except for the Fife peninsula and the Eastern coastal fringe, the weather is a lot different to anything they would have been used to, and then there are the midges...
If one takes into account that Hadrian's Wall was designed less as a defensive line and more as a customs and control line. It allowed control over who entered and the land beyond was dominated by mobile cavalry patrols. It was a line held by auxiliaries with the main heavy infantry being in Eboracum (York).
So if the Land between the two lines is productive it makes sense to trade with it and tax it whereas if the land beyond the Antonine Wall didn't bring in the same results economically then HW makes more sense. Also in terms of political control, tribes just beyond HW varied from allied to hostile depending on tribe and time, so by diplomacy one could ensure a certain amount of security which I think was less likely beyond the AW.
So I agree with you that the effort to maintain the AW probably wasn't worth the resources, or the midges.