Could the answer to the questions that you have posed be that even enthusiasts later than the "Ian Allen generation" are looking for more than a "standard 4 + Mk 1s experience, and that restoring older stock gives a heritage railway more interesting stock? I could foresee a situation where wooden coaches become easier to maintain, in some ways, than Mk 1s and there may be a case for scrapping Mk 1s currently in use of favour of restoring wooden bodied stock. Beclawat window frames, for example, have a nasty tendency to break when removed to deal with the corroded steel they are attached to and I shudder to think what a new casting would cost. Railway preservation is not necessarily about cost, though and whilst railways rely on volunteer support to keep their coaching stock fleets restored and maintained it will have to be recognised that those volunteers may well have interests beyond Mk 1s You can lead a horse to water, but you can't make it drink! If I went to Pickering and was told I had to work on Mk 1s rather than teak carriages then I woldn't bother going.