I believe that the thinking of the past that "plate modulation is the
only way to go" has more to do with not coming to grips with the fact
that it is about how much power we can put into the antenna and not
how much plate input we run as in the past.
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Question : Do the new FCC rules, eg 1500 W PEP mean ,transmitter output
power or power to the antenna , the latter meaning that any feeder
losses may be compensated by a higher max. transmitter output ?

The latter is the case pertaining to the UK amateur radio licences.
With a maximum output of 26dBW = 400 W PEP (on most bands) , in the case
of 3dB feeder loss ,the permitted transmitter power is 800 W PEP.

Frank GM0CSZ / KN6WH (who has NOT read as yet the 'new' FCC max
power rules)