not sure if I'm understanding what you are comparing with. what is your alternative?
if we are taking authz as a need in your org and also a single point of failure for your entire services (as you said in your comment), are you suggesting you would instead build / host your own Zanzibar like authz service, and that your in-house solution would do better in terms of availability than this external solution?
leaving aside how much you'd lose on productivity, I would guess your in-house solution will break more unless you spend a lot of time making that a highly available service (and you'd depend on other SaaS for that as well, which may also break).
Also, if it's in house / on prem you can do a change freeze and actually mean it. With a SaaS that's a joke. You never know what the SaaS engineers might be doing. An in house system's downtime is more likey to be correlated with your other systems' downtime, so total downtime is going to be higher with a SaaS, especially since authz/authn is so critical to everything.
I don't understand why this is a SaaS and not just a software product you buy and run on prem. (In terms of business risk, buying this as an on prem product seems perfectly reasonable -- after this startup is bought by someone like Microsoft that can actually guarantee the features will stay around.)
if we are taking authz as a need in your org and also a single point of failure for your entire services (as you said in your comment), are you suggesting you would instead build / host your own Zanzibar like authz service, and that your in-house solution would do better in terms of availability than this external solution?
leaving aside how much you'd lose on productivity, I would guess your in-house solution will break more unless you spend a lot of time making that a highly available service (and you'd depend on other SaaS for that as well, which may also break).