Honestly don't think expansion would do a great deal to reduce L2 gates at all. Increased competition rarely leads to increased apathy.
If you're minded to go and watch a middling L2 side anyway, an Accrington, a Gillingham or a Fleetwood say... is it really any great difference playing a, say, Rochdale, Scunthorpe, York or Hartlepool when the 'cream' of L2 this year were Bromley, MKD, Cambridge and Salford?
Boreham Wood would be small but over the piece there really isn't a major difference in club sizes between the upper end of NL (thus those likely to be promoted) and L2.
Plus of course with 3UP there would be increased jeopardy in many more games towards the end of the L2 season, that only increases gates.