I am prepared to give a certain amount of leeway to physical shops in not having the exact variant of product I'm after.
e.g. cycle products. Years ago I wanted a new derailleur - went into JE James, not the smallest most independent cycle shop in the world. Asked for a 10 speed, low normal, short cage, shimano, MTB, rear mech.
No.
OK, forget low normal. Have you got any 10 speed, short cage, shimano mechs?
No, sorry.
OK, have you got any 10 speed, shimano mechs?
Doesn't look like it.
Have you got any shimano mechs at all?
er... no.
About 6 years ago, probably even worse now.
I'd personally be prepared to go to an actual physical shop if they had a reasonable range - after all, sometimes with amazon you get too much choice. But the range of products in physical shops diminishes to just the ones with the most margin purely because of the rise in online shops, so it's not just because online is more convenient. It's just not possible any more.
To an extent I’d suggest that it’s also partially about the manufacturers of bike kit riding the wave of new riders. They continually “ improve”, new model each year, poor backwards compatibility, combined with a new interest in “ tech”.
When I started in the 60s a Bike would be current for over 10 years. Campag gears lasted for ever, and if you’d crashed a new one would work fine.
Now, as for your Shimano rear mech there are a big number of variables over the last 5 years. Stock in a shop loses money fast, and there’s usually a huge penalty for the shop if an order isn’t a minimum value. Lots of shops order from Wiggle.
As has been said, the future is in service. Whereas in my youth having your bike mended, let alone “ serviced” in a shop would have been a matter of shame. Many of today’s riders see this as normal.
Shops that get this can thrive, but hanging onto a 2005 part just in case, isn’t business sensible.