The fees are to pay the miners…
As a trivial example, imagine you have 1.5MB of transactions competing for a 1MB block, let’s say this results in (as roughly it does today) 3 BTC of fees in a block. That goes to pay the miners and is essential to their revenue.
Now say we instantly double the capacity to 2MB, now the 1.5MB of transactions have zero competition and everybody pays the minimum fees. Now there is close to 0 BTC of fees. That is a shock to the fee market, and would upset miner revenue, possibly resulting in them losing money and needing to shutdown until it becomes profitable again.
It looks like the linked proposal would actually raise the limit by a few bytes in each block, so there are no sudden jumps that would upset this balance. There would probably be just enough space to accept 1 extra transaction every 10 or so blocks of growth.