On-Air TV jobs haven't declined because the model isn't, nor has ever been the same as radio. Back in the day, there were typically five shifts held by at least one DJ or host per shift. For many years TV local talent staffing amounts to news anchors, street reporters. weather, and an occasional part time morning traffic person. Many of the TV control rooms are automated, requiring only a Director and Producer.
Radio can be, and is commonly voice tracked during entire shifts, so your midday, night, and overnight shift can be tracked from home, or via talent from another market. Many stations do the syndicated tracks at night from folks like Delilah and Tesh. That sort of workflow doesn't work for TV...Yet.