Ducati bevels were never a great bike until Paul Smart and Bruno Spaggiari managed to do OK in a couple of endurance races. After that the SS series became more desirable but the electrics and build quality were far from Japanese level. They created a racing history and image and then A certain Mr Cook Nielsen ran Old Blue onto the box, followed by guys like Jimmy Adamo and so on. There were not a whole lot of bevels made and even less running or reliable which is why they are mainly in collections rather than being ridden.
While not exactly false this is more than a little misleading. I realize your endurance race comment was tongue in cheek but to the uninformed, the bevel 750 SS was Ducati's first twin and won it's first race out of the box. So Ducati didn't have to wait long for Smart to "do ok" on one. That bike created the Super Sport series for Ducati and the public had to wait two years before they could buy a street going version of it. The race didn't make it more desirable, it didn't exist before that day.
Anyway, Smart's result at Imola didn't "create" Ducati's racing history. It brought it to a new level of course, but bevels had been winning races for decades as small displacement singles before the twins were debuted. The go fast versions of the bevel singles are also collectible/valuable, though not to the same level as the '74 750SS. That said, those singles formed the basis for the new twin.
Someone mentioned bevels are rare due to their cost when new. That's not entirely true either. Ducati has always been a small company. Not to be Captain Obvious but they are rare because not many were built, which is also one reason they have always been a little more expensive that Japanese bikes. Until recently, Ducati could only build 40k bikes per year. They had one factory (disregarding Mototrans for simplicity's sake) and that was it. A few years back, Ducati built their one millionth motorcycle. That same year, Honda built it's 300 millionth motorcycle. Both companies have been building bikes for roughly the same amount of time. It's not like Ducati could have built 300 times as many bikes if they charged less. They physically could not build a lot of bikes, regardless of cost.
More bevels in collections rather than ridden? I'm not so sure that's true. A shitload of bevels are still being ridden. '74 SS's? Hell no, but how many people do you know that are riding $100k Japanese bikes? The bevel twins make great riders. They are only as unreliable as the owner allows them to be. In the years I had my '74 Roundcase (A GT, not an SS) it virtually always started first kick (unless the kill switch was off, but I digress...) and only let me down once - when a ground connection I had built failed. FWIW - contrary to popular belief, most bevels are NOT desmos, Ducati didn't go to Desmos exclusively until 1980 or so. My bike, like many bevels, had rockers with screw adjusters. Setting the valves didn't require any shims or magical talents. High maintanence? Ok. Highly unreliable? Not a given, depends on the owner.
Anyway, they're 40+ year old bikes at this point. Any issues they had when new have well known fixes at this point. The 750 bevels are known to go high miles with no issues if maintained. The larger bikes have more fragile lower ends but if not abused (lugging them is worse than revving them) they will last too.
I'll give you they do not tolerate abuse like a Japanese bike, and there are few secret handshake type things that are helpful to know, but the bevel community has a rabid fan base and surprisingly good parts supprt (if you aren't stuck on having 100% originality for things like textures of surfaces, etc).
The problem with bevels is they've been discovered. When I bought my 750 I paid $750 (it was pretty rough). Five yers later that same condition bike would be worth closer $5k. I spent a little over $10k rebuilding mine and still made money when I sold it. I enjoyed riding mine a lot but could never get over what other bikes I could buy for the same money. I sold it and used the money to fund other toys (including an 851). I still have a couple bevel singles around as project bikes but will likely never have another bevel twin.
THAT rave/rant/ramble out of the way...
I'm not sure what people consider "collectible" to mean. If you like it, is it collectible? Or does collectible mean prices get out of reach? As already noted, as an investment they pretty much all suck, save a few rare examples.
20 years isn't really long enough to define a collectable IMHO. Nothing pedestrian from 2000 is collectible right now. Sure there are super limited production whatevers that are but nothing that the average Joe may have bought in 2000 is all that desireable right now. But I suppose the OP meant 20 years from now regardless of model year? And things that aren't already expensive right now? There may be some '90s stuff that is starting to catch on - sticking with Ducatis I think the '90s Super Sports are starting to go up in value but will never catch fire. Maybe the Super Light in the USA, more so than the rest of the world. The 851/888 may catch on (I hope so, I still have mine!). The 916 series gets talked about a lot but save the homologation specials they haven't caught on either. I had a very clean/original one owner (me) low mile 2000 996 monoposto that I sold a couple years back. Very little interest in it and I didn't price it that much above the going rate for a molested version.
Aside from something like sandcast 750s, CB whatevers and XS650s never were and never will be the holy grail BTW. Think bigger than that. RC30s aleady got away (DAMN I wanted one of those, should have looked harder for one in the 90s!) so what's the next Honda to take off? It won't be something they made hundreds of thousands of. It won't be something everybody had, it will be something everybody wanted but bought something else instead for whatever reason.
