BA F cabins are not always full and I have found that when the are, the service levels go significantly down as the limited crew numbers struggle to cope. Meal services takes ages.

CX have the same strict limited upgrade policy.

Flew once with CX with 4 pax in F (parents, me + ANO)
Flew once with SQ with 4 pax in F
Flew once with MH with 1 pax in F (just me)

Bad for airline profits but great feel of premium value for the pax given the attention although at times I found it was to much. On the SQ flight, they sent 1 FA to make 10/15 casual/friendly conversation with each pax. Got the full tuck into bed option when the bed had been made up. On the MH flight, they were hoverring to top my champagne glass every time I took a sip (a full bottle of DP just for me).

Even SQ F crews struggle when they have a full F cabin.

Suppose it is about justifying the premium price for pax who paid it; although some recognition of brand loyalty would be good. Suppose, as a business they feel that it is built into the miles redemption scheme.

Its the US airlines that upgrade to a full F cabin - but then the service levels have never been that good in comparison. Same issue.