It is also very important about which tables you are talking in regards of upgrading. some are worth it, some not. Which apple are you comparing to which pear?
I read a couple of stories from guys that dismissed their heavily modded VPI's in favour of (massively modded) heavy plinth Lenco L75's. Hard to grasp for me but very interesting since I'm gonna start modding my own Lenco's (an L58 to practice first) next year.
From what i've been reading and know about turntables the heavy platter/plinth Lenco route (DIY mods) and modded SL12xx (upgrade mods) route are very interesting..
SL1210
(This used to be a) Lenco L75
Ps VPI's aren't too shabby by themselves. (From what I've read, haven't heard one yet

)
You raise some interesting and valid points.
A heavy-platter Lenco can still be found today for a reasonable price. Get a Pete's Top Plate, and a massive CNC-cut slate plinth, perhaps a titanium idler wheel and a nice tonearm, and you'll have a "killer" quality TT... but at a price. Still worth it, in sonic terms and in ratio to what you spend. If you don't want to spend that much, you can use the original top plate and a laminated plywood (CLD) plinth, keep the original (metal) idler wheel, and do all the work yourself, and get almost as good a sound for a lot less money. Definitely one of the bargain routes to high quality, and probably well worth it.
Kabusa makes all kinds of optional upgrades for the Technics 1200 series TTs. They make it a lot better, but they also add up in cost. I'd say it's a toin-coss whether you get your money's worth, if you go whole hog on one of these. If you go to re-sell it, you may not recoup the total expenditure.
Similarly, Linn LP-12s have a long list of "recommended upgrades." Each one enhances the sound, and upgrading turns a decent TT into a very fine one...but is it worth it? Maybe, maybe not; it depends in part on what you are looking for sonically, how you value your money, etc... At least, with the LP-12, there is a well-established resale market, and you can make a reasonable calculation and decide for yourself. And you are putting your money into a TT that people are willing to pay a fair amount for, so perhaps the upgrades are worthwhile.
I bought a GT-2000 a little less than two years ago. Since then, I've put money into a better (uncracked) dust cover, original manual, a new tonearm with the auto arm-lift, a separate power supply, better underpinnings... and I'm still looking for more upgrades, ready to spend money on whatever original accessories may cross my path. I don't worry much about the cost, for two reasons: (1) I got the TT originally for a low enough price (compared to market value) that even after spending more, I'm not at risk of losing money, and (2) this TT is good enough to justify significant expenditure. I don't expect to ever re-sell this TT (unless, perhaps, it is to upgrade to the GT-2000X), but if I decide to, there is a ready market for it. It's hard to imagine making this TT any better than it already is, but I know it is possible, so I'll do it if I can.
Would I spend money on costly upgrades like a massive plinth or costly mat, etc... for a Garrard Zero 100? No, because no matter what I did to it, there is a fairly low limit on what anyone would buy it for. If I were deeply enamored with that TT, and wanted to keep it for my lifetime (as I feel about the GT-2000, even if I later get an even better TT), than maybe I wouldn't mind, but it's hardly worth it economically --or sonically!
There is no simple answer to the original question; the most accurate answer can only be given on a case-by-case basis.
I think you need to consider:
1. What you paid for the TT.
2. The cost of the upgrades.
3. Your "emotional attachment" to, or preference for, the particular TT.
4.How you would feel about the alternatives available.
5. The resale value of your TT.
6. The sonic quality of your TT before the upgrades.
7. The sonic quality of your TT after the upgrades.
8. The quality of alternatives at the price level before your upgrades, and what they would be like after any comparable upgrades.
9. The quality of of alternatives at the price level after your upgrades.
10. How the alternatives mentioned would fit into your existing system.
11. How the alternatives mentioned would fit into your future sytem plans.