I think the original TM-D700 was released around 1999 and the TM-710A was introduced in 2008. The only thing the 710 lacked really was built-in GPS, which they added in the TM-D710GA.
There's always a chance the 710 could get replaced but I wouldn't consider that as an important criteria for selecting a ham radio unless there is specifically a feature you really want or expect the radio will get. There's nothing really in the radio that needs corrected so the model will be made as long as it continues to sell and components are available to build them. The inability to get a key component or a suitable substitute is the most likely reason Kenwood would discontinue it IMO. Adding a digital mode is the only other reason and that would be a new model. Is that something you need?
If Kenwood does bring out a new model tomorrow it wouldn't be something I'd personally buy immediately anyway. The first batch of any new model always has production glitches so you'd want to wait a year to see. Then buy in and sell the TM-D710, which will sell easily for years to come and will get snapped up no problem on the used market. It's a very popular radio.
Heck, I watch casually for an Icom IC-7000 to replace one I sold some years ago. But when they come up they sell quickly and not all that cheap, often as much as a new radio. So some models are I guess like the Tacoma of the ham radio world, inexplicably valued over others.