Actually, the limit specifically prevents any disagreement over what constitutes a valid block in regards to size. Without that limit, the entire network must trust all miners to never disagree on the size of blocks they are willing to validate and relay.
You dont need trust
all minners use the same maximum block size they are willing to validate and relay, because there will be always only one longest chain called Bitcoin
No there won't be. "Longest" doesn't mean anything on its own. The only thing that matters is the "longest
valid chain." Validity is determined by consensus rules. Even if there is no consensus on a block size limit, that doesn't stop anyone from rejecting blocks on that basis. If one miner accepts that block and builds on it, and another miner rejects it, "longest" chain loses any meaning. The latter miner will never accept the longer
invalid chain.
thats the beauty of PoW and selfish interest of miners to build only on top of longest chain, otherwise their just building worthless chain and miners will quickly learn the hard way to dont build worthless chains for whatever reason, like limiting only to 1 MB.
If a majority of miners fork the rules to add 84 million coins to the supply, who is building on the worthless chain? The ones who broke the rules, or the ones who stayed on the original chain? Okay--now apply that to your example.
You should not be afraid some miners might temporary build on top of smaller chains because of their software having disagreement over what constitutes a valid block in regards to size, because they will learn the hard way by producing worthless blocks, thus they are encouraged to build on top of longest chain only next time (after they realize their mistake).
What evidence do you have that there could not be more than one worthy network to mine on? As long as it is profitable, a rational miner will do so.