Could CHECKSIGADD be used on new ECDSA multisignatures instead of CHECKMULTISIG?
No because the opcode 186 (0xba) is undefined for SegWit version 0 spends. You can only take advantage of this opcode in SegWit version 1 spends. If you included this opcode in your SegWit version 0 transaction script any attempt to spend it would encounter this script error.