No, are you? Because you’re the one suggesting this change here. This is what YOU want right?
If you’ve played the game enough (multiplayer especially), you might think more realistically instead of casually saying, “Ah this is a weird design that must be changed”. And no one, absolutely no one in their right mind would delete Wonders, Forts or Factories unless there is a super rare and desperate situation where their units need better pathfinding or need to be prevented from being blocked entirely.
Even if we apply this argument to Manors (which is the core identity of the British civ - Spawns a settler), your suggestion will not be able to make them deletable before reaching the max build limit (like other buildings) and also they act differently upon being rebuilt. It is a weird design by definition but still, your brilliant suggestion is almost like asking Germans to stop sending uhlans with every shipment for a while because they take too much pop space, Ottomans to stop training vills and Russia to be able to queue single unit in the blockhouse instead of a batch because every other civ can. It’s that absurd.
Once again, I also think manors should be deletable but it’s just not simple to implement. Don’t you think so many people have thought of this before you? They didn’t react because it’s never been an issue.