Cheaper and easier to upgrade the constellation to newer and faster tech. If you have backwards compatibility, you just start launching v2 and v1 will eventually just burn up, and hopefully finish just in time for v3 to start launching so you only have to be compatible with n-1 versions.
Especially with the number of them it’s probably cheaper to just put up new satellites. LEO sats are designed to be temporary.
Cheaper and easier to upgrade the constellation to newer and faster tech. If you have backwards compatibility, you just start launching v2 and v1 will eventually just burn up, and hopefully finish just in time for v3 to start launching so you only have to be compatible with n-1 versions.