Why would anyone want to force the blocksize limit expanding at pre-set dates. Now, give a sensible reply to what was intended.
1. To give enough headroom for adoption and rise in transaction volume.
Answer the whole question, not the part you prefer. Why the pre-set dates and limits? Why can the size limit not decrease when required under this scheme? How can these projections, and the arbitrary stepping that manifests them, possibly hope to conform to reality over the next 18 years?
2. To disincentivise spam attacks. (More saturated the block is the more profitable it is for large miners to team up and make spam transactions in order to drive the fees up.)
Rogue developers have a similar incentive to launch their own spam transaction attacks under the same circumstances. Their payoff is development control, and of course, all such people would need to do subsequent to the success of such an attack would be to implement a feature that prevents any future hard forks to wrest control away from them. Such a feature exists in the XT roadmap (checkpointed, aka centralised, chain consensus)