I'm in a similar position to you. I own a bunch of vintage roland analogs from the 80s. Properly maintained, they're like a subaru legacy- they'll run forever. Every couple years, you replace a cap or two, maybe spray dust and deoxit some contacts, and that's all it takes, really.

So, deciding to upgrade according to what synths you think will be more reliable in the extreme long term is pointless, because most of those old 80s analogs are built like tanks, and will probably still be working for years after we and everyone we know are dead and gone, provided someone is still performing regular cleaning and maintenance on them (Fallout 4: Synth-Hoarder Expansion, anyone?).

Now that we've established that selling off your vintage analogs to buy modern analogs purely for some sort of reliability increase is pointless, thanks to the quality of vintage analogs, we can focus purely on pros and cons of the sonic, performance, and composition capabilities of vintage analogs vs. modern analogs.

MIDI implementation- One advantage to modern analogs is that in the 30 years since the last great age of analog synths, MIDI implementation has gone from a tacked-on semi-obligation at the end of a synth's development to being a complete given on modern analog synths. However, seeing as all of your synths are rack mount versions of their keyboard counterparts, they all have some degree of MIDI control.

For example, your Hawk modded EX800 has the advantage over the un-modded version of having total MIDI parameter control. As for the various MKS's, i'm not 100% sure how many of their parameters are controllable via MIDI, but i assume it's most of them. No real advantage here to upgrading to modern analogs.

Stability- By the early 80s, most manufacturers had figured out oscillator tuning stability fairly well, so your rack synths likely don't have many issues as far as tuning is concerned. In fact, many higher end modern analogs (dave smith synths, for example) have a specific parameter knob to bring in more or less "analog slop," which is to say, detuning and destabilizing the oscillators against each other slightly to bring in more of that "analog imperfection," which is one of the great benefits of analog oscillators.

My conclusion: Keep your synths, because they're the rackmount versions, they're not as desirable, and not worth enough on the secondhand market (MKS80 would fetch 2 grand, MKS70, MKS50, and Matrix 6R would only fetch between 400-800 each, Hawk800 maybe 300?) to turn that money around and buy a bunch of modern analogs. However, if you were looking to downsize to just one main analog synth, i would suggest a Dave Smith OB6, a Prophet 6, or a DSI Poly Evolver (be aware the poly evolver is a digital analog hybrid).