Apple cannot add functionality that the hardware won’t support.
There are other ways to NETWORK and log into one Mac from another (big letters may help reading comprehension).
There would have to be a compelling reason for Apple to build Target mode into a future Mac. You’ve not given any reason other than it’s something you want. Ok, you want what you want but... It has to be something that millions of others would want and something that Apple Marketing would be able to tout as a desirable feature.
You haven’t even attempted to make a case as to why a user like I would want to ask for the return of Target Mode.
So, why in the world would I want to shut down and reboot a target machine so that I could control its drives from another when I can do it simply by having both machines on the same network, wired, wireless or over the internet?
I used Target mode for years to diagnose and repair Macs. With PPCs, it was a useful tool but it had to be done one at a time. Nowadays, I can log into an entire classroom and look at 25 computers simultaneously, clicking on each one from my laptop to see what I need if there’s a problem. At the beginning of the school year, I can reformat all campus classroom computers to the same, standardized configuration with key stroke. BTW, nearly all of these are old enough to support Target mode as is the laptop I use. I probably used it last in 2012 to update a 2008 iMac from my 2010.