(i wont comment on blocksize change anymore - unsure whats best and i'll keep the decision to the people who have to decide it: miners)
Sorry for repeating myself, but you are bringing up previously refuted points:
The miners decide, they can be influenced by points 1 and 2, but they decide
For point 1, if 80+% miners leave (if not, fork won't happens), it will be long to recover and have a difficulty that fit the left hashrate.
For point 2, it's economical factors that will influence miners. so they still decide, but one can influence them. also, to buy on Bitcoin chain, it will be very hard to move coins with the left hasrate.
(I call Bitcoin the actual Bitcoin chain)