It just needs UX improvements, not necessarily UI. And it should be achieved through the mythical EpochTalk (that's pretty much a dead project now I think lmao).
UX improvement will probably bring new UI elements and design changes, I usually don't write UI and UX together and I believe these words are often abused use only UI but probably you are right.
In a business sense, that's going to be next to impossible unless Theymos' takes a huge risk through making a huge change on Bitcointalk — which I doubt he would do.
I'm afraid dramatic changes are necessary for better future of this forum. To be honest, I think that at the moment signature campaigns are a huge drive of this forum but this is not a long-term solution if forum is unable to attract new customers in upcoming years. If there are no new customers and some old people are leaving, then there won't be new campaigns because it won't be financially and statistically beneficial for companies to pay here. And if sig campaigns die, I'm afraid this forum will die too, that's why dramatic change is necessary and theymos has to take this risk if he cares about the forum. I also think that community here is very friendly and educated unlike other forums, so, he may get help from some users.
Therefore, it is not the structure of Bitcointalk that is outdated. At least we can say it's the layout, the design. Despite this, I have nothing against this design - even though part of my job is making websites. Maybe it could be suitable for mobile devices? Yes, it could... but if we look closely, that is also part of this forum and what makes it different.
Yes, layout is outdated and probably being a forum alone the exact way it is right now won't be enough in future.
By the way, it's entirely possible and very easy to make this forum responsive. This forum is made via HTML tables and we need to convert it in CSS Flexbox or CSS Grid. Then we have to convert some images in CSS grid and that's all. I don't know what kind of changes he has to do in SMF CMS but I think it won't be hard.
Here is an example of how would this forum look like if it was responsive: bitcointalk.org/index.php?topic=5414229