It’s the worst possible time to be stepping away from a works engine deal. If McLaren play it smart, they’ll engineer a swap of engine with Toro Rosso, with a contractual clause that will allow them to swap back to Honda for 2019 if Honda sort their shit out next year.
Alonso is a positive for McLaren, but at thirty six it’s unrealistic to expect him to be around in 2019. Better to retain his services for an official works Indycar challenge (or even Le Mans) with a Honda engine.
By 2021, with new regs it’s not unfeasible that McLaren could partner the likes of Porsche (who built the TAG engines used until the end of 1987) or even build their own power unit.