ESA already has at least three other major projects (the Venus probe, the much-delayed LISA, and something else) wherein NASA is, or at least was going to be, a junior partner, so if ESA has to pick up the slack on any joint projects, it'll probably prioritise those.
(LISA is a particularly ridiculous case. It was pitched in _1997_ as a joint ESA/NASA mission, was finally picked up in the noughties, NASA pulled out in 2011 due to budget cuts, LISA was cut down and then cancelled in favour of JUICE (which itself was a redesign of _another_ joint mission that NASA pulled out of), then LISA was brought back a few years later, with NASA as a junior partner, and an MoA was finally signed last year. Now the NASA side has apparently been cut, though the ESA side is continuing, and who knows, maybe eventually it'll be launched...)
Honestly, ESA should probably consider just stopping doing joint missions with NASA; NASA is just too vulnerable to political interference.
(LISA is a particularly ridiculous case. It was pitched in _1997_ as a joint ESA/NASA mission, was finally picked up in the noughties, NASA pulled out in 2011 due to budget cuts, LISA was cut down and then cancelled in favour of JUICE (which itself was a redesign of _another_ joint mission that NASA pulled out of), then LISA was brought back a few years later, with NASA as a junior partner, and an MoA was finally signed last year. Now the NASA side has apparently been cut, though the ESA side is continuing, and who knows, maybe eventually it'll be launched...)
Honestly, ESA should probably consider just stopping doing joint missions with NASA; NASA is just too vulnerable to political interference.