"Great" is always tricky, as then you're talking about something exceptional.
Those things are too rare to really follow any distribution curves.
Sometimes they're there, sometimes they're not.
But the number of good bike mechanics is probably directly proportional to the number of people being willing to pay what it actually costs for a thorough service.
As an example:
A colleague of mine asked if I could help him sell a bike he had that wasn't being used.
"Sure" I said, "bring it over".
Closer inspection revealed a mid-80s bike with very little mileage and mostly stored well.
And a very mid-80s color scheme.
Although a good-quality bike from the start, by now it needed all bearings repacked, all cable sheaths replaced, etc etc.
Nothing actually difficult in any of these, only a decent number of posts on the task list.
as an example, let's look at the pedals.
If repacked, the original pedals would probably last thousands of miles and/or another decade.
But I can get some basic department-store pedals for $5.
It'd probably take me maybe 30 minutes in total for pulling the pedals off, taking them apart, clean them out, count out and install the right number of really tiny ball bearings, etc before the pedals are back on the bike again.
And the bike might sell for $50-80.
Apart from the warm, fuzzy feeling of a job well done, and with a minimum use of resources(other than time)
there's simply no point in doing the thorough job.
It wouldn't make the bike more sellable. Or up the price any.
Or cables.
On my own "nice" bikes, I use stainless cables, with the ends kept from fraying by silver solder/braze.
Takes (proportionally) a lot of extra time, but sorta handy every now and then.
Now, unless my shop is set up assembly-line style, with one station standing ready to solder-seal cable ends, setting up doing the job and stowing away the tools, the flux the solder etc, will probably add another 20 minutes of work.
How many would be willing to pay for that?
I'd LOVE to work for someone who said "do this to the best of your ability, and I'll pay you a liveable hourly rate for it".