I think this is a good starting point for estimating share of externalities in a start-up (particularly the expected externalities that will be caused if the start-up is very successful).
I don’t think it will be all that accurate, for the kind of reasons you mention, but it has the major advantages that it is easy to measure and somewhat robust. I expect that replaceability means that it tends to be an overestimate, but typically by less than an order of magnitude.
A warning, though: replaceability can operate on the level of startups as well as the level of jobs. You should consider that if your start-up weren’t very successful in the niche it’s going for, then someone else might be (even if they’re a bit less good). This will tend to make the externalities of the whole company smaller than they first appear.
I think this is a good starting point for estimating share of externalities in a start-up (particularly the expected externalities that will be caused if the start-up is very successful).
I don’t think it will be all that accurate, for the kind of reasons you mention, but it has the major advantages that it is easy to measure and somewhat robust. I expect that replaceability means that it tends to be an overestimate, but typically by less than an order of magnitude.
A warning, though: replaceability can operate on the level of startups as well as the level of jobs. You should consider that if your start-up weren’t very successful in the niche it’s going for, then someone else might be (even if they’re a bit less good). This will tend to make the externalities of the whole company smaller than they first appear.