This is a great question.
I agree, and I guess that the "mojo" is something everybody experience in a different way, probably for small things like some different nuances in the pickup sound, some different measure in the neck profile, a particular color.
for example, I've had 5 different Stingrays over the years, two of them were Standard, three were Classic: the last one I bought, a 2004 Standard, didn't felt much different from the others, except for the slab body of the Classics (also I don't find much difference in the 3-EQ or 2-EQ, small nuances I can avoid...), but it has something...
it weights around 4kg, perfectly balanced (as every Stingray), the neck has some flame and it slept 20 years under the owner's bed, so the unfinished wood was immaculate, and I oiled the neck so that it shows it's beauty, with some flame.
also it's a trans-red coloured body, my favourite color ever, and this "combo" of specs just made me fell in love

it was then strung with GHS Brite Flats 49-108, and it's a real keeper.
70's Fenders were kinda frowned upon, now they're the hottest thing ever. When did it change or is my memory just wrong?
as far as I remember, it's not "70's fender", but some specific periods. let me explain:
early 70's Fender, used on many records back in the days, and coming from the near CBS passage (1965/66) were still considered good instruments with "value" from the Fender brand.
you can recognize them because the body is still alder, the rosewood is a nice chunk of wood, the tortoise pickguard is always gorgeous, and the finger-rest was still under the G string.
the "bad period" started (for "common knowledge") in 74/75, up until the early 80's years, before Fender started producing the "reissue" models.
in the second half of the 70's CBS started using ash (mostly not dried enough and too heavy, to produce more in less time and less money), lowered the quality control, started using overwound pickups, and ironically created a series that now is looked for because it was particular and with a different sound
in that case, when you find a late 70's Fender that weight less than 5kg, it's already good, if you were looking for a bass with a middier and sparkier sound.