It's a niche "industry" that will be ok as long as the mania with "collectors pricing" continues in some regard.
Provided you're in an area with enough monied people who want pins and are interested in high-margin skilled restoration work it might be sustainable for a good while. The problem comes with trying to actually make a decent living at it; the amount of time that goes into true quality restoration work, especially when you have to start fabricating things on a bespoke basis, gets out of hand fast when someone else is paying the check.
There ought to be a hell of a price gap between "players" games (those that display ordinary wear and tear from time and use in a commercial environment), "players+" games (those that were either refurbished -- not restored -- after coming off a route and have little PF wear, all electronic/mechanical issues fixed, but cabinet fade, etc) and "collectors" games (those in new or new++ condition that have been *restored* including cosmetics.) The labor to do the latter, even with a "HUO" game or one that became HUO before playfield wear occurred is tremendous. Those sorts of games will always carry a premium price and should but the candidates for that sort of treatment are few and far between with any sort of economic justification and I suspect the clientele who will pay those prices are quite few in number. Consider that if you put a man-week of labor into such a restore 40 hours of your time @ some reasonable skilled labor rate (e.g. $50) means you just tossed $2,000 into the machine -- now how many true restorations, not "touch up, shop and fix" jobs, can you do with one man-week of labor? (ha!) Is that labor input, plus the cost of the parts (or their fabrication) at a rational mark-up, recoverable on a sale? It had better be or you've got an unstable business model. Further, in a given area there may be some decent amount of service revenue available from those who own pins but cannot (or don't want to) maintain and repair them on their own *and* are willing to pay a good amount of coin to have someone come out and maintain or fix them.
Today there has been a lot of compression in the pricing and it's almost all been from lift on the bottom, which is flat-out backward and is why I often comment that the current state of the market is ridiculously frothy. When, not if, this corrects the used machine market for all but fully-restored (that is, "new++" condition) machines is going to fall apart on price; I fully expect to see 75% falls *or more* for a lot of titles from where they often sell now. If you're in this business and not in any dependent on that pricing being maintained to stay alive then you have a potentially sustainable environment. You just have to be quite careful that you're not predicating your business on the continuation (or worse, continued escalation) of used-condition games in the marketplace; IMHO you need to be getting essentially all your revenue on the service side either restoring machines or servicing them so you're insulated (or would even benefit from!) a collapse in price of games in the used marketplace. IMHO that correction is inevitable simply because of the impossibility of earn-out given current new pin pricing; you have multiple unstable elements that are allowing the current manufacturers to get away with this and the one thing every entrepreneur knows is that change is the only constant in business.