Oh you can upgrade a solo machine alright.  Bill owns (owned?) a solo max2.  I don't think there were a lot of those machines to begin with, and they probably just filter out as they get old and break and such; not to mention they don't have high demand.  I don't know why I answered your post with no insight on anything, sorry.

I saw a Solo 4th Mix (or may have been 5th Mix) a few weeks ago in the game room at Lincoln Square Cinema in Bellevue.

Unfortunately:
1) it's off with an OUT OF ORDER sign
2) someone put the safety bar on backward > >;

What? Wouldn't that make it stick WAY out from the machine? I mean, the solo bars were bent inward, weren't they?

From my personal experience with them, Solo machines just didn't feel like they were constructed as durably as the regular 2P machines. So wear and tear could have taken their toll and have caused many of them to be removed or even trashed. T_T

It's simple!  We had our solo, and I know of a handful of others that were here in the U.S.  Like ours, they have all been surplus saled, usually ending up in the home market.  They are in peoples garages now.
The kids dad who got got ours and shipped it back east phoned us awhile back to thank us, and said his kid had been looking in to how it was hacked to a max 2 and still had all the inputs as well as lights actively working.  Pretty cool, they are having fun with it.

It's great to hear that machine went to a good home.

On one hand, the 6-panel layout did required a bit less structure around the the top 5 arrows, so I wouldn't be too surprised if they didn't hold up quite as well as normal machines - plus they only used 2 central sensors per arrow instead on one on each edge, which would make them more prone to errors (individual sensors take a lot more of a beating, so they'd wear out faster and then there's less redundancy as a sensor dies), which would cause lazier operators (who do little maintenance, if any) to scrap them.  Maintenance concerns paired with a lack of upgrades (yes, you can force normal versions onto them without too much trouble, but most operators probably wouldn't think to do that) would lead many arcades to drop them faster than a full size cabinet.

On the other hand, Solo machines do hold up fine if maintained, cost much less (comparatively), and have a much smaller footprint, so it's not surprising to see Bill mention that just many made it into homes - I wouldn't be surprised if more home machines were Solos than normal machines until a few years ago (since you can't have a Solo machine if there aren't any left to buy, plus the private market has shifted to ITG and I don't think that many people have tried rigging up an ITG Solo cabinet, since there'd be a bit of modding required to get the pads working well enough to play ITG charts - bracketing doesn't work very well with central sensors).

Either way, given the significantly smaller production run of Solo cabinets in the first place, it wouldn't take that many private buyers or scrapped machines for them to generally disappear from public locations.

Especially in the U.S.