Reason for existence of 2.5/5G does not have that much to do with the cost of PHYs themselves (it.s more or less the same circuitry as for 10G) but with power c

Reason for existence of 2.5/5G does not have that much to do with the cost of PHYs themselves (it.s more or less the same circuitry as for 10G) but with power consumption. Even 1000-base-T is surprisingly power hungry due to the fact that the used signaling is essentially analog and not some simple binary/ternary line code, which necessitates inefficient analog final Tx amplifiers and such things. Probably all current 1G PHYs contain some TDR-ish magic that measures the length of attached cable and determines how much can be the Tx power scaled back in order for the link to still work (~10 years ago this feature was often the main point of manufacturers' advertising, today it is essentially must-have). 10G PHY is even more power hungry and 2.5/5G is way to not only scale Tx power back but to get additional power headroom by reducing the symbol rate.