but I wonder if there ever has been any discussion or similar concern for a client supermajority in the Bitcoin network?
Yes, even Satoshi expressed his opinion about that:
I don't believe a second, compatible implementation of Bitcoin will ever be a good idea. So much of the design depends on all nodes getting exactly identical results in lockstep that a second implementation would be a menace to the network. The MIT license is compatible with all other licenses and commercial uses, so there is no need to rewrite it from a licensing standpoint.
Bitcoin Core's 98% dominance.
Note that:
1. Even if someone uses some modified version of Bitcoin Core, most node runners do not bother to change the default client name into something else.
2. If someone changes that name, it is usually done in brackets, so it is still identified as Bitcoin Core.
3. There are different versions of Bitcoin Core in use. Some article about it:
https://blog.lopp.net/when-do-bitcoin-node-operators-upgrade/4. Even if someone is running his own client, it usually is still connected to some Bitcoin Core node, just to be sure, that it will be "bugward-compatible". Because sometimes you may feel a need to "fix" something, and then find out, that your "fix" is actually a hard-fork (and then, you are for example mining bad blocks, which are always rejected; or you don't accept the strongest chain as valid, because you reject for example a block with transaction containing SIGHASH_SINGLE bug).