They would also need to take responsibility for any security issues related to the chipset. It is also not possible to upgrade proprietary firmware (e. g. for the modem) at all without support from the chipset manufacturer.

Fairphone doesn't seem to care much about security (they use public keys for signing their OS afterall!), so they may be fine with those compromises.

Qualcomm is interested in selling new SoCs, so even if they actually offer support extensions, their fees are most likely very high to make it unprofitable for manufacturers to go this route.