This is a good point.
My strunal ply is more than enough for all my pizz needs, fills the room at jams, is easy to play and is virtually indestructible. but sounds horrible under the bow. When I get frustrated or fatigued with arco I play my ply for fun.
Take both your Strunal Ply and your carved bass to a great luthier for setup optimization. Optimize the Strunal for pizzicato /(slap?) playing and the carved for Arco / pizzicato playing.
Really, diminishing returns in not a point, but a curve. It happens right away. More money needed to get fewer returns.
But if you need the performance, you spend the money.
IMO, if folks are performing with a pickup and amp, basses over 10k USD in year 2022 prices are nice, but not needed. Amplification will be the limit. (sweet range= $5k to $10k)
For acoustic only players with a bow, they need more. But the curve is still in place. Ever decreasing improvements cost ever increasing amounts of money to get.
Then there are the regular economics of supply / demand.
Add in the economics of heritage value, and folks with money simply wanting something, whatever the price.
Also, an upright bass/cello room in a shop, should be optimized to make instruments sound even more fantastic, so customers want to buy. Wood paneling, nodes in the desired place, reflections, reverb, all favourable to help sales. It won't sound that way at home!
Consider money spent and money coming in. An aspiring orchestra player seeking a union job in the $100k/year range, needs to bring more to blind auditions than does a performer in a club or concert hall being amplified or in a recording studio.