The DfT is alleging it is to save money on maintenance.

Which isn't quite right. The real reason is to save money on
alterations.

There are also many, many items (e.g. avionics) that are FAA approved
only, and for which the European approval route is expensive

Bingo. But that European approval route is what keeps much of the
European GA bureaucracy in business. Ditto those required courses for
the PPL and IR. Ultimately, people do keep planes on the N-register to
save time, hassle, and money - but that time, hassle, and money they
don't want to put up with is the lifeblood of the aviation bureaucracy
and the people who make their living in it. Of course those people
want the practice to stop.

Michael