J&J and AZ adenovirus-vector covid vaccines are highly novel vaccines that are very similar to mRNA vaccines, just with a different delivery mechanism.
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.