IF the 6th gen catapult capable jets achieve FOC, say around 2032-2035, China would have 4 to 6 aircraft carriers in operations. There is no need to phase out J15T since minimum air frame lives is usually more than 20 years in normal use.
Phasing out an aircraft before their air frame lives become expeire is extremely rare in PLA. We are still seeing aircraft designed in 1950s in active service in China in 2025.
Old aircraft still in service are JJ7, J7, J8, J10A, J11A, SU-27UBK, SU-30MKK/MK2, JH7, Y5, Y7, Y8, K8, Y12, JL6, older variants of H6 and IL-76 (bought second hand and refurbished).