bitcoin 50% attack as absorbing barrier: stable control of all mining by one entity (double-spending is irrelevant)

When bitcoin experts talk about a 51% attack, by which they mean control of majority of hash power by one entity, they always talk about double-spending attacks, as tho that's the thing to worry about.

Controlling majority hashpower means the controlling entity can include or exclude whatever blocks, as well as transactions, they want.  Someone else solved a block?  That's nice, until that's not the main chain anymore, because controlling entity replaced it with a longer [0] one, which by definition they can do sooner or later.

The bluntest strategy by controlling entity would be to exclude all blocks solved by others.  Tho sustainable in a system where nothing else changes, such a strategy would bring about dramatic second-order effects, from directed investment in balancing hash power, to the total failure of bitcoin, or perhaps changing the proof of work element.

A more subtle strategy would maintain an edge by reducing the probability, say by 10 percentage points, that a block solved by a competitor is included.  When all the variance is taken out, bitcoin mining is a low-margin activity, and a monopoly with a 10 percentage-point advantage would continue to dominate.  The competitors would just not be able to make ends meet, and from time to time one would go bust.

Is anyone monitoring metrics around blocks solved and broadcast but not making it into the main chain?

[0] "longer" has a specific meaning relating to the core invention, blockchain + proof-of-work, in bitcoin