Not quite. The resources we're saving are due to limiting the complexity of the voting system. By freezing discussions, topics automatically go out of date and cease to affect processing of new rewards. Only the small minority of topics that people discuss over a long time will be reposted.
I agree with your second point, but it wouldn't be particularly challenging to have a workaround for this, such as pages for post series. They could highlight where posting is still possible or just have you post in the latest by default. You could even build a front end view which amalgamates continued threads into one continuous thread. I doubt it's a high priority issue yet though.
What if after the 30 days rather than increasing the complexity of the reward mechanism, you just disabled the reward mechanism and let the discussion continue?
Building a new front end to steem to maintain a single topic is outside the skillset of the majority of steemit's userbase.
You'd probably need to check or ask on GitHub why they didn't go with that option.
It wouldn't be for one topic. If it was just about one topic, it wouldn't be the major disadvantage you said it was. If there is a demand for this feature, the Steem Blockchain and codebase is quite open for someone with the skills to fill this demand.