I think PvE would be much more interesting if the essential characters needed were of more variety. By making the essential character either the same across the board or the latest release makes it impossible for many to progress in these events. It would be much more satisfactory if you had to have multiple characters essential throughout the event.
This would require people to manage their rosters more so they keep a lot of characters, as you never know who you might need. It would also make these events have a more level playing field. In a 5-day event who wants to learn on day 1 they have no chance of progressing since they “don’t have that ONE character” they need?
Heck…it might get some people to even keep bagman around…