In fact somebody else already requested this:
Personally I would find this very useful. I frequently switch between chats and manually scrolling to find the part of the chat I was reading is very time consuming, so if Delta Chat remembers the position in the chat instead of scrolling to the bottom every time, I would welcome that.
From a user perspective I think options are very useful for helping people with different workflows or different preferences make the most out of the app, but if it requires a lot of code to implement or maintain, I can understand why it might not be worth the effort for developers.
If it is better to avoid an option for this, I think it makes sense to not automatically scroll to the bottom because as you point out, it is easy to do this by tapping the scroll button.
By the way there is another feature proposal which is similar but not quite the same, which proposes keeping the position in the chat instead of automatically scrolling to the bottom when a message is sent:
I would also find this useful, since I often refer to messages higher in the chat when writing messages and need to keep scrolling up after every message I send.