> Why would anyone that already ditched C asserts use C++ contracts when it can’t express what the alternatives already can?

You have not understood the C++26 contracts proposal.

It introduces three new syntax viz. "Pre" and "Post" in function signature declaration and a separate "contract_assert" which you can use anywhere in the code path.

It is the "Pre"/"Post" that is useful here. Since it is part of function signature declaration, a static analyzer can easily make sure that all function calls satisfy the contract else you get warning/error. The compiler on its part can inject appropriate code at the function boundary points to do runtime checking etc.

You cannot do this with assert macros. If you wanted something like this earlier, you would have had to change the compiler to emit modified function prologue/epilogue and build your own contract framework which few people had the capability to do.

You can now see the immediate benefits particularly for standard library (and other) code hardening needs. Many UB circumstances can be easily flagged at compile time without too much complexity.

Finally, note that if you use compile-time switch "ignore" to turn-off everything, only the runtime checks are elided (so you do not lose any performance) but the static analyzer can still catch all possible inconsistencies at compile-time thus giving you the best of both worlds.