The financial logic of what Carmel is doing centers around zero-sum competition with neighboring communities. Carmel is betting that by being the nicest place with the fanciest amenities, they will attract the richest families and be able to support that heavy debt burden in the coming decades.
This is explicitly not a model that every community could follow! It relies on spending more than would be prudent for the current tax base, in the hopes of standing out in comparison to other communities. And for sure its a bet that could fail badly, depending on general economic conditions. Its too soon to tell if the gamble will work out successfully, and very hard to tell even in hindsight how risky it was, succeed or fail.
[1] https://www.point2homes.com/US/Neighborhood/IN/Carmel-Demogr...