It is possible as long as the implementations are fully compatible with the Bitcoin protocol. However in the case of a change that is fundamental to the protocol, which makes previous versions incompatible with the new version is a hard fork. Increasing the blocksize will require a hard fork whether it is done by Core or an alternative implementation. Hypothetically it is possible if there was enough disagreement that Bitcoin could split, there would then be two Bitcoins essentially. This should not nesserally be seen as a 51% attack but more as an act of freedom and expression of personal beliefs. This avoids the problem of the tyranny of the majority. Cryptocurreny as a whole will therefore always remain free as long as enough people choose freedom.
Yes, tyranny of the majority is bad. In order to avoid this, everyone should strive to achieve consensus, instead of persuade others to comply with his idea, everyone should try to find a middle ground that is acceptable by as many community members as possible
Sure, everyone has the freedom to make his own fork and see if his idea realize. But it is a big community, miners, exchanges, payment processors, investors all hold stake in the ecosystem. If you don't go with major consensus, then almost for sure your fork will become another alt-coin that no one cares. Although theoretically you have the freedom to fork, but you can't achieve anything without following the major consensus
Another reason: Why there are thousands of alt-coins but none of them matters? Because in order to adopt a deflationary monetary system, you should only focus on one most mature cryptocurrency with limited money supply. Dilution of the resource to other coins is essentially an inflation in cryptocurrency world, which benefits no one economically