There may be multiple valid concerns about license proliferation, but the concern about introduced license incompatibilities is certainly one of them. The experimental Copyleft.next license document is already compatible with all present and future versions of the GNU GPL and GNU AGPL and all licenses compatible with those licenses (with one marginal category of exception). One GNU GPL-incompatible license I'd like Copyleft.next to achieve compatibility with is the Eclipse Public License 1.0, but I think this will require an approach somewhat like that taken by MPL 2.0 or by GNU GPLv3/AGPLv3 to achieve cross-compatibility.
The marginal category of exception is the set of GNU GPLv3 + limited badgeware licenses; Copyleft.next is currently designed to remove compatibility with such terms. A notable example is the license of SugarCRM Community Edition.