This is purely theater/political in nature. If they are pulling their AD to the beachhead of the Kerch bridge, all they are doing is putting them into a position to pull them over the bridge to the other side of the border if Crimea starts to fall and attempt to keep them out of harms way.
There is no real functional AD design that would provide greater protection for the bridge by pulling the AD back to the bridge vs being used as an umbrella over the rest of Crimea. All this does is move them away from protecting the airfields and naval installations at Sevastapol while hoping these very expensive units can't get reached by HIMARS etc. This also signals that Russia knows full well that they can't defend the Crimea from the air and all this is an attempt to keep the Kerch bridge open as a viable line of retreat for equipment at the cost of putting a serious strain on the ground defense in both the Kherson and Crimea Oblasts.
I would be real curious what the latest imagery shows regarding troop density and fortifications over the Kherson river and around Armiansk in particular. If you are seeing a general withdrawal in the secondary units in the reserves on the Russian side of the Kherson River towards Armiansk and Melitopol, it is an obvious signal that the Russians believe that they cannot hold the Kherson Oblast when the F16s show up.
The problem with using natural barriers, like the Kherson River, to limit the Ukrainian advance is that it also means that you end up with a heavy density of troops massed along those barriers. If you look at the geography of the region, its a funnel in the Kherson Oblast that leads to a land bridge in Armiansk to be able to bring troops into Crimea proper from the north. There is really no other way, beyond a legitimate amphibious landing, to get to Crimea proper. On paper, this approach will tie up the Ukrainian forces and make it not worth the attrition needed to mount a breakthrough and in essence allow Russia to hold the Crimea. So Russia can pull back their troops and place and dig in a blocking force at Armiansk.
That is fine when you have enough AD to control the skies. Now you have lots of equipment that is vulnerable and no AD umbrella. So either you entrench them and hope for the best or pull them back to the next layer of the AD umbrella,. The problem with that is now the umbrella has moved down to Kerch as an epicenter and so you lose the benefit of geography if you want to pull your blocking force back under the AD umbrella. So in short, if they feel that they can't hold the banks of the Kherson, then they are going to have a slow delaying action back to Armiansk and leave their main blocking force to the mercy of the F16s and HIMARS. To me, that is an untenable situation that will become a bloodbath.