SMS is (unfortunately) core to the product, so I'm not certain how they could make it cheaper, while retaining the same properties (user+pass registration would be a nightmare for spam and change the UX).
From a footnote of the article. Maybe this is why they've stayed with "infinite scale, infinite costs" (commonly known as "cloud") so long? Surely at some point this is worth considering though, I would also be curious where that point lies
Virtually anyone, also when spending only 100 euros/month on server providers, can save a large percentage of costs by taking it in-house. There might be a gap where you need dedicated personnel and it's briefly cheaper to outsource before you grow and it inverts again, but generally if you've got a stable service then this is nearly always worth it
Maybe a hybrid, where new users onboard onto cloud and they buy hardware for expected loads (i.e. current users), would be the most cost effective. I wonder how hard that is to combine the two worlds, but anything that requires more than one server already has that sort of communication going on so there shouldn't be any real blockers. Maybe the two types of infra add costs/risks again and that's why one rarely sees this setup?