For the local roads, a full line closure would need a massive amount of buses. The local road network would not cope. Youre comparing apples with pears. No motorways. An A road with in places single track, that wouldn't cope with huge number of coaches or buses that would be required. Just to Lelant a much reduced number would be needed.
The closure also clashes with the hugely popular St Ives food festival.
They will by busing passengers 0.8miles.
https://goo.gl/maps/kYiVtFThjA62
The first half of which is a mixture of the A30 that has recently been widened and improved to allow for the new P & R carpark at St Erth, down to the St Erth roundabout, where they will turn down a reasonable A road to the next roundabout, where they will then turn into the approach to Lelant Saltings.
I do not understand where this "single track A road" is, nor where these huge number of buses and coaches are going to be required.
And considering that my daughter manages a coffee shop in St Ives, which happens to be a venue during the St Ives Food Festival, and that she has heard no complaints about this, plus the Council and others have all been aware of this for a long time, I does appear that mole hills and mountains come to mind.