767ER bottle discharge
"Company SOP’s state U cannot open cargo doors before disembarking pax. "
I was wondering about this and found that the second and third bottles "discharge thirty minutes later, or at touchdown, and are metered into the affected compartment to maintain the required concentration of extinguishing agent."
If it has been less than 30 minutes since you first pushed the discharge button, you are going to get fresh agent in the selected cargo bin right at touchdown.
Perhaps it was determined that this would provide at least equal effectiveness compared to what ARF (airport rescue & firefighting) could offer by opening up and doing their thing. It may also have been a general statement meant to cover all situations, allowing some discretion by the Captain.
This is one of those rare "fast" emergencies where time could mean everything. It is a critical decision whether to evacuate or taxi in.
It is a safe, sound, and supportable decision to evacuate. The risk of taxiing in to the gate might be an unacceptable risk.