Every single thing on this planet changes, upgrades, whatever name it.
Why not accept BU? Because some folks are going to lose the capita?
Why don't they thing that we need faster, cheaper BTC transactions in the future?
For me it's 100% normal to switch to BU like in a decade we must change to the new BU version 2 and going on...
BU will never be accepted by the community because it is a centralized company trying to do a hostile takeover of Bitcoin. Core are a volunteer bunch of developers (some of whom are paid by MIT and Blockstream) but any developer can volunteer to work for core and help make Bitcoin better. BU want in install a president of Bitcoin, not all the code they release is reviewed, not all the code they release if visible for anyone to see, and all developers are paid employees. So if BU ever manages to take over the Bitcoin chain, Bitcoin will lose 90+% of its value within 2 years, people will move to independent alts. Who wants a company managing Bitcoin?
So Core developers have released an update called Segwit to improve scaling. This should allow about twice as many transactions as currently allowed and fixes a known bug, but it paves the way for lightning networks which will provide off blockchain transactions for some Bitcoins transactions that are instantaneous and should cost less than 1-5 cents.
Segwit has already been tested and released, it needs support from miners. They are worried about Lightning network so most are not supporting it and many are supporting BU instead.
BU's bigger blocks can also mean smaller blocks, the miners decide on the blocksize, so in theory they could vote for smaller blocks and thus higher fees. Whilst BU promise lower fees. The other issue with larger block is the larger the block the more decentralized the miners become. If the blocksize went much larger quickly only data centres would be able to mine as no one else would have the bandwidth or storage needed. If the blocksize were to hit 4mb in the next year or 2 then the whole of Austrialia and Africa would be unable to process the data needed to mine due to bandwidth. So that is 2 continents without any possible mining.
Larger block sizes still has some merit but BU has none. If you are after speed, and cheap fees going offchain with LN is the way forward, and Segwit is needed to get to LN.
BU is not dead, and if it dies someone will come along and try and fork Bitcoin.
Last point this is all or nothing. If there is a hard fork to BU then BU has to spend all of its resources killing off the Core chain (they have a $100m kitty for doing this). BU have to do this as if they let a core version of Bitcoin survive everyone will use it. BU is paid internet trolls in USA and a few Trump supporters mixed in with some big Chinese miners (who may be trying to kill Bitcoin on behalf of the Chinese government, or may just be greedy.)
If the Core implements big block, there is no need for the Unlimited. I think Core should be more open.