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Your mistake is that you think these are "upgrades" when actually these are simply changes each targeting arbitrary things that have actually added vulnerability to the otherwise secure protocol.
Eventually everything is an actual upgrade of Bitcoin, even Bitcoin itself.
The Segwit implementation/upgrade and block size debate has split it in 2:
Bitcoin and BCH without none really being the Original Bitcoin anymore.
So yes, upgrades, modified versions, whatever it is.
Their value proposition is evolving slightly along the way.
Not the network but the "blockchain" is immutable. And it is not time-dependent, it is timestamped.
Timestamp is a form of time dependency (time-stamped).
The network is "writable" only for a 10 min window after which it becomes immutable.
I disagree, what gives anything value is their utilities meaning bitcoin value comes from its utility as the decentralized currency.
But Bitcoin isn't yet optimized for this purpose: High transaction time & fees, network congestions, no privacy, no other use cases (utility) for the network like ETH or EOS.