Let me relate my experiences as a sole trader with work coming out of my ears and not a clear day to take on anything else for at least 4 weeks, I even have to work Sundays sometimes to clear the previous weeks backlog, , I do very nicely thank you and it is because I know where my limits are, I carry several thousand pounds worth of diagnostic gear as I think probably a lot of garages do but unless you have the volume of that type of work coming through the doors mr average trader is always going to be struggling with the electronic diagnosis side of the game.
Because I know what I can and cannot do ,if a for example, japanese MIL light customer comes to me I tell them straight that I cannot deal with their problem, but I know a man who can!, also luckily for me,he is in my vacinity,(and a member of UKAT), he would not do a clutch on a mondeo I would, so I think this is the direction that most of the majority of the trade would take, even in motor mechanics , we can be the jacks of all trades and masters of none !
Horses for courses, I think specialism will probably be the way forward,in whatever field you are good at,