Logically and practically, new MK1's will be needed though, in the longer term , as they age and ever more costly rules become more stringent, which they will. No regulation ever goes in reverse, it just increases step by step. LSL are the most likely to do this, to ensure a long term future for railtours with vintage stock or, new build 'old' locos with new build 'old' stock.
One possible long term outcome of increasing regulation is the total banning of old locos and stock, in which case only new build locos and stock will be allowed on the main line anyway. In that scenario, we will then see new A3, A4, MN, Castle, Duchess, Deltic, 40, 45, 52 etc. The new LMS diesel 10000 will rule the main line. In the long term. Think beyond the here and now.