[MobiusHorizons]

I think you are missing at least one important point. The reason the mechanic can’t work on the other brand of car is not knowledge or skill, but equipment. It costs money to buy the full suite of equipment required to correctly service a particular manufacturer’s vehicles. It often makes much more sense to specialize and make more use of fewer expensive tools than to have tools for everything and have only marginally more business.

[Dah00n]

Well, that's a very American way of doing things that most places simply don't do. The norm is for tools to be compatible with all brands and at best it is an added option to unlock or a pay per use. You can do 99% of the diagnostics with a Wish Bluetooth dongle and a free android app if you wish, since by law it is an open standard.

Most specialty equipment costs less than a mechanic can earn in a day. You even order the parts from the same company no matter if the bumper is for a Mazda, VW, or an Alfa. Or a Kawasaki motorcycle for that matter. This lock-in behavior is, luckily, mostly illegal.

[MobiusHorizons]

Diagnostics yes, but try to reset the warning light on a German car after fixing the fault. This can apply even when replacing seemingly purely mechanical parts (I think some suspension parts on bmw/Mercedes). You will need vag-com for any vw/Audi and something else proprietary for Mercedes or bmw

[Dah00n]

I have only done so on a Seat, with no trouble. Seems to me that if this is true it is against EU law.