That's a bit of an oversimplification. Soft forks add new rules, yes, but those new rules can offer all sorts of previously unavailable transaction types and templates, provided that they are still within the original consensus rules.
It is in this way that Segwit can not only create a new transaction type which addresses transaction malleability, but can also increase the block size without breaking consensus.
IMO its never good to restrict base protocols - adding 1000s of lines - technobubble them up. Client impls should do this.
Open source protocols needs to be as simple as possible. Or you stifle things (client imlps, knowledge, adoption,..) to dead. That's exactly what we witness right now and too much alts come and share bitcoin's realm. I'm still a bitcoin maximalist and do not believe only big blockers 7 big bitcoiners do see this.
Sadly this witnessing has been already segregated away due to form politics / big blocker have left and you need to look it up elsewhere. Here (or reddit/bitcoin) you cannot much discuss this in peace and I'm just waiting to see what will happen to good old bitcoin. Still hope for a compromise 'most' can agree on and the first step of this (HongKong,..NYA ) was already done.