Very much agree. I want to emphasise again that the fact that a skill is lacking in a region (native-level English proficiency, love and attention towards the children) doesn't mean that the net impact will be positive; I mentioned the sense of abandonment once volunteers leave and the possibility of overreliance on foreign aid, and surely there are several other hidden side effects. But it hints that there is real value that can be added by non-locals, and that the challenge then is to maximise it and minimise the counterforces.

There's something else I haven't discussed in this post. It can be that, once you are in the location 24x7 for 3 weeks, and your job is to teach in the mornings and you have the afternoons free, it can make sense to build houses or put up windows in your free time. Especially because the vast majority of times the reason why football pitches aren't created or the facilities aren't improved is because the people managing the funds aren't interested in it or don't take a wider view despite the clear benefits of the action. In my case for example, the school director preferred us to give them the money directly (which they were planning to use to buy phones or laptops for the teachers) rather than us adding the windows (which could prevent the kids from catching frequent colds).

Volunteers in this case clearly appear net-positive: they invest time or money to provide value in ways the natives with power for decision and action wouldn't be able to recognise. So that's good. Even better perhaps would be if the volunteers identified a need (e.g. to add windows in classrooms), paid a local professional to do so competently, and the volunteers instead used their time to provide their non-replaceable skills by playing with the kids, giving classes or lectures, applying this outside view to spot opportunities for intervention…

That's a good argument I hadn't considered, about how it opens the window for interest in EA. It would be interesting to know how many came to EA after volunteering abroad.

Also, as you said, many times the money spent on volunteering would have been spent on other leisure activities such as non-volunteering trips. Which means it usually can't be net-negative in terms of the allocation of money; it can still be, however, in terms of the impact it has on the developing country.