Well, many Georgists believe that All Taxes Come out of Rents, so under that belief there is no reason to keep inferior taxes in existence as opposed to collecting the higher rental value that would occur with the removal of those current taxes.

As for levies on externalities, these should be more accurately seen as correcting a market failure, the social cost that society must pay that the consumer/producer do not pay.

Notably, Frank Ramsey and A.C. Pigou can be considered crypto-Georgists -http://blog.lvrg.org.au/2013/09/ramsey-and-pigou-crypto-geor...

"As we shall see, Ramsey not only formulated a rule that leads directly to a “single tax” on land, but also anticipated the so-called Laffer curve in cases where the “single tax” is not employed. Moreover, Ramsey's rule was to be applied after any externalities had been internalized by means of appropriate taxes and bounties."