Which part of "RANDOM" do you not understand?
Even the most generous rolling ability can potentially end up with a scenario where you have only 1 score above 15.
All that means is that despite the DMG's advice, you've ended up with only one score of 15+. Or none*, if you're really unlucky.
Play it.
Who knows, you might beat the odds. You wouldn't be the first.
Some of the best and longest-lasting 1e characters I've seen started with one good stat (15-18 range) and a bunch of average (10-12 range or lower). My longest-lasting 3e character - 6 years, 14 adventures - started with one 15, the rest were all 12 or less.
* - disclaimer - for a long time I've used something similar to the suggestion given in 3e: if you have nothing above 13 or if your average is less than 10, chuck it and start over.
As for other rules affected by their era:
- if magic items had not been given all sorts of interesting and wierd abilities to start with, I don't think they'd be getting them now; the trend since has been toward genericization and synchronization with existing spells (e.g. 3e wands and potions only duplicating spells, or 4e items on-body location largely determining the type of ability they may have).
- the proliferation of the internet and instant information exchange and access has led to a tightening of all the rules; where houseruling most of the game was once the accepted norm, such things seem to be slowly falling out of favour except for minor tweaks as people seek (and are encouraged to seek) homogeniety.
- monsters-as-PCs have been around for a long time (Drizz't, I'm looking at you and wishing you'd go away) and while they've been more supported by the rules of late I'd not be too surprised if in future we see that support backed off a bit.
- Birthright as a setting would have done much better had it come out in the pre-1e era; it could have presented the political backdrop to the wargaming roots of 0e. As it was, it came out about 15-20 years too late.
- a DMG today would not have a wandering prostitutes table in it, more's the pity. That said, I wonder: had the BoEF been better done might it have started a trend toward more "adult" (i.e. R- or X-rated) themes in the game?
Lan-"what stat does a brazen strumpet need a 15 in?"-efan