Maximalism is the mindset that your project is the only valid one.
It depends, how do you want to define it. Do I think that Bitcoin is the only valid project? No. Do I think that it is good to create separate monetary bases, and abuse 21 million coins limit? No.
Bitcoin is not the only valid project, but altcoins should not create their own coins, out of thin air. In case of Bitcoin, when it was launched in 2009, there were no other altcoins, so there were no reasons, to attach to other monetary bases, like dollars, because they were strictly incompatible with "digital cash". However, since 2009, you don't have to make new coinbase transactions outside of Bitcoin blockchain. All you really need, is to make regular, non-coinbase transactions, and extend existing use cases, without inflating the supply.
Bitcoin is not inherently superior to other cryptocurrencies like Monero or Litecoin; it simply has its own trade-offs.
Imagine how better could Litecoin be, if you would have just some P2Pool-like 2.5 minute blocks (or 30 second blocks with 20 times lower difficulty, as in the original protocol), and if every Litecoin transaction could be batched and notarized on Bitcoin, long after being confirmed on Litecoin.
And imagine how better could Monero be, if you could have just some single UTXO for Monero, with some assigned amount, and you would have just one-input-one-output, moved from time to time, with nobody knowing exactly, who owns what inside Monero, while staying perfectly auditable in BTC. You could have just a single signature, moving the whole coin forward, and hiding from the outside world, all details about amounts and public keys, which would be hidden inside.
Many altcoins were created in the way they are, because people didn't know, how to write a better code. The only reason, why people switched from SHA-256 into something else, is because they didn't implement Merged Mining properly: if they would, then 51% attacking their coins would be as hard as 51% attacking BTC. The same with Monero: it has "fake multisig", because people didn't think about pubkey-based commitment at that time.
Being a Bitcoin maximalist is actually contrary to the spirit of Bitcoin, because it involves unwavering support for Bitcoin, when you should be skeptical and rigorous in examining its potential weaknesses.
Note that being a Bitcoin maximalist doesn't mean, that if we would launch the network today, we wouldn't change it. You can see it, when you look at recently launched networks: all soft-forks are activated from the Genesis Block, without unnecessary glorification of legacy ways of doing things. If not limited by backward-compatibility, we would start new test networks with P2TR-only addresses, with all OP_CHECKSIGs working on Schnorr signatures, without SIGHASH_SINGLE bug, without duplicated transaction IDs, with witness stack in all transaction inputs, and so on.
If we will ever be forced to do the hard fork, then the new network will be started with all of those fixes. However, because we have around 600 GB of history, and 15 years of transaction data, we cannot just throw it away. However, as things will be simplified over time, and we will have for example purely UTXO-based model, like utreexo, then maybe we will reach a point in the future, where all nodes will be started in a new way, without handling a lot of legacy code.
Don't be myopic and dogmatic. Say no to maximalism.
First, try to say no to "abusing 21 million coins limit". If we will get there, then we can think about maximalism in the first place. In the past, when people launched new altcoins, they were told "Why are you doing that? You can do this on Bitcoin, too!". Now, I can see people saying the opposite, like "Launch an altcoin, because it is not going to be merged into Bitcoin". And this is bad in the long-term, because then users cannot have feature A and feature B. They have to sell one coin, and buy another, to get more features, instead of just using existing coins in new ways.
Having no maximalism is like being forced to buy vouchers for each and every store, where you want to go shopping. The end result is that instead of having dollars, BTCs, or some single and established coin, and buying things, you have a lot of useless vouchers, and you have to trade them on a daily basis to buy anything (and each trade costs you more money, than you would spend, if you would have all your savings in a single coin, used in different ways).
Another reason to have a single coin is speculation-related: in the long-term, many altcoins died, or lost their value over time. If they would be just wrapped BTCs, then you would have 1:1 peg, and they would never be "dead". You can check, what was LTC/BTC in the past. Or XMR/BTC in the past. Imagine having LTCs as wrapped BTCs from 2013, and having 0.03 BTC per LTC now. Or imagine having XMRs as wrapped BTCs from 2017, and having 0.03 BTC per XMR now. Sounds like a great deal, right? But no, you said "no to maximalism", and instead you have far worse exchange rates, and you have to sell your BTCs, to get some XMRs, and make some private transactions.
If you don't want maximalism, then what is your alternative? Hard-forking every time, when you want to make some big changes? Or maybe burning all BTCs, to get brand new BTCv2s, with new features? Or maybe selling all old coins, to get new altcoins, with new features? In the last case, it seems like the market price does not encourage people to do that, in the long term.