https://www.hsph.harvard.edu/news/multimedia-article/were-be...
This seems to be at least partially supported by the current risks in regards to currently available non-mRNA vaccines (such as AZ).
Also I'm not really sure what saying "we don't know the long term effect are" accomplishes. It's not really adding much to the conversation.
It doesn't seem like it really matters if adenovirus vector vaccines is an "old idea" - after all, the idea of using MRNA has also been around for a long time. But if we've only started actually using them recently, how different is our knowledge of long-term effects?
The fact that it is an "old idea" is indeed unimportant, but what is very, very important is that large scale study over long periods has already been performed on that vaccination platform so we really do know a lot about it.
That means that the similarity to mRNA is a good thing in terms of any worries about unexpected consequences. Of course it also means that we know that the risk of any unexpected consequences for adenovirus vaccines is also very low.
You are making the claim that there may not be any long term effects to the vaccine, but the information you cite has nothing to do with mRNA-based vaccines. I'm simply suggesting we don't have the data yet to make such claims.