Overhaul or Replace?

My two cents: there's no reason to say "enough is enough". All the things you have had to replace are essentially consumables. Yes some last longer than others, but they all wear out through use. But then you replace them and you're back to being as good as new. The only time I would say it's not economical to repair would be if there is structural damage to the frame. 

Furthermore, while I could see the appeal of upgrading to something newer, I'm not convinced a 90's road bike would solve any of your problems. Although it might appear shiny, it can be very hard to tell without inspecting the bike in person what state of repair it is actually in, so you might buy the bike and then have to cough up the same again on repairs. Furthermore, a 90's road bike is likely to have components and standards that are no longer commonplace, making it harder and/or more expensive to source replacement parts. And even if it's in perfect shape now, you're not solving anything long term - the components on the "new" bike will wear out just as surely as the components on the Moose.

I would also question whether a 90's road bike is likely to be as suitable for workhorse duties - does it have mounts for mudguards? Pannier rack (if you use them)? Would the position be as comfortable/suitable for urban riding? 

In my opinion any all weather day to day bike should have disk brakes. Rotors are a lot cheaper than wheels. Hydraulic brakes are self adjusting as well. As for the drive train it's not a difficult job to do yourself and sourcing parts is straightforward. You will need a couple of special tools to remove the cassette but they will last a lifetime so it's a one off cost.

I've just gone for a 3 speed disk brake bike as my run around. Hoping the chain will last a lot longer than the 8 speed I've been using for years. Early days yet but so far so good. I don't have to do any steep climbs in my everyday riding, if I did I may think differently.

So if you want to stick with the bike get the bits and do the job yourself. The wheels, I am afraid, will still cost an arm and a leg.

Treat yourself to a new (or new to you) bike.

Personally, I'd think that a 90s road bike is a bit too old in terms of technology as it'll be older than your current one. A second-hand MTB is worth considering as well - slap some slick tyres on it and it'll be great for commuting and possibly cheaper to maintain than a road bike.

The trouble is, without knowing the history, mileage and maintenance, that 90s road bike could end up still needing similar maintenance.

A couple of warnings, a 90s road bike is going to have very old shifters, e.g. Shimano RSX and thay are unmaintainable. Campag of that age was maintainable but it is very difficult to get hold of parts. If you want to replace, then you may be lookiung at a new rear wheel depending on the freehub.

At that age it is almost certainly a square taper bottom bracket. It also is likely to have a massive front chain ring - it is before the days of compact chain rings, though if you are lucky it might have a triple. CHances are it has some high geared rear cassette too.

Also, road bike design has come on a long way in terms of comfort and design. If it is aluminium you also have questions as to whether the frame is sound - steel lasts, aluminium is more likely to have stress fractures.

Braking performance can be very variable on old bikes, again, without knowing what is on the bike, you may be getting something that simply doesn't stop as well as a nicely adjusted V-brake.

Maintaining a bike that is comfortatable and has worked well for you is not to be dismissed. I'd say if you had seen a more recent bike that was in known good condition and you were using maintenance to make a contribution then I'd consider it, but I think going that old is not wise.

Road bikes do seem to command a premioum over hybrids, and although there are good reasons why road bikes can be better rides for longer distances, I think I'd consider what other bikes are knocking around before going for a 90s one.