Also, for long haul, you won't get by without dispersion compensation, so the lower noise from the higher signal levels getting into the amplifiers helps you.

There are limits to the amplifier output power due to how tightly single mode fibers concentrate the beam: if you'd send 1W through a 9μm core, you'd have about 1.5 MW/cm² power density. Even carefully packaged laser diodes break around 12~20 MW/cm² (according to Wikipedia; "catastrophic optical damage"), not to speak of open air patch connectors in a fiber hut.

Don't forget the fiber carries around 100 channels that share this power limit.