Following Abraham Lincoln's "I can make a brigadier general in five minutes, but it is not easy to replace a hundred and ten horses", an alternative would be to accept a higher risk of losing the President and having to failover to the hot spare, in the weird and unlikely circumstances that the AF1 measures would actually help. Thereby saving a few billion dollars.

Zelensky seems to be doing OK in a helmet and flak jacket. I did check whether he has a presidential plane - the previous president was a notorious big spender - and the answer appears to be a leasing co. https://en.wikipedia.org/wiki/Ukraine_Air_Enterprise based at Boryspil airport southeast of Kyiv. "Temporarily closed" on google maps.

I think it's not just about keeping the president alive, but also about keeping the president functional. A president who's alive but trapped in a plane without good communication isn't very helpful to the country. In a time of emergency the president needs to make decisions and communicate them quickly.