5 levels in about half a year means 36 levels in about 3-and-a-half years. Then what?
I guess your definition of "a lot of legs" is different than mine; at your current advancement rate you're looking at less than 2 years total to get to 20th, and you can spin it out for a while after than with Boons and so forth. In my eyes (as player) 2 years into a campaign just nicely gets it started; I've learned a bit about the world and what the main stories are, I've probably gone through a few characters before finding one or two with enough luck and-or starch to stick around, and I'm ready to see where it goes for the indefinite (as in, many years) future. (caveat: I'm talking about table games here, just noticed your example is an online game)
Then what - if I did actually get a group of 36th level PCs in BECM D&D, which seems unlikely,
and they wanted to keep playing, then I'd get hold of the Immortals set and do BECMI.
The alternative of course is new PCs - this is a pretty traditional campaign which allows
for multiple PCs, my son (8) insists that all his main PC's children are also his characters
- "I rolled them!".
But realistically, few tabletop players stick around that long here in London, my
Karameikos group are mostly young and their careers will likely take most of them away from London within three years. In my 25th level 5e D&D campaign only one original player & PC is still there from the
start at 1st level in 2011, three more of the current group joined through 2012 at ca 6th-8th,
and the fifth joined at the start of this year at level 22.
Re the 5e Wilderlands game that might reach 20th in a couple years, actually ideally I would like to play a lot of Epic stuff at 20th level so I could see it going a long time at 20th, but eventually
the players will want new PCs - this gets in to what a 'campaign' is separate from a 'world' since there are NPCs in the current game that some players have known in prior campaigns from 2-5
years ago, I think my first online Wilderlands campaign was 2009 and there is player
continuity from then.
Finally, in my big 1e AD&D campaign that went for many years, by the end the PCs were demigods and a lesser gods and we had years of deity-level play.
Overall, I don't think slow advancement is necessary for continuity unless you don't want the game to change, in which case I'd prefer a lateral-advancement system to D&D; BRP as in
Pendragon & Runequest maybe. But I really like in D&D seeing the campaign change
over the years, zero to hero to superhero to legend to demigod in some cases. I don't much like 3e/PF super-fast advancement though, my favourite 3e campaign was Lost City of Barakus
using half XP.
Edit: Overall I think WoTC were right to think that most campaigns last 1-1.5 years, but wrong to
design 3e around going 1st-20th in that time - they designed 4e to go 1st-30th in 1.5 years but
failed IME since combat is slower than they realised. I would say Gygax was right to design around 1st to name level (ca 9th-11th) in a year of weekly play, that is a satisfying campaign for most people. And he
was right to have slower progression after name level for the long term campaigns that wanted
to stick with the same PCs. 5e by contrast has a similar rate to 10th level (quicker 1-4, similar
5-10) but then rockets you up from 11th to 20th. The difference though is that 5e power curve
seems a lot more shallow; I was reading the 5e DMG yesterday and my 7th-8th level PCs are still
doing the sort of
adventures it describes for the level 1-4 novice tier, I suspect at 20th they'd mostly be doing
the sort of thing it describes for level 11-16 paragon tier, and that should work fine.
