It is not implausible to recognize that the definition of "generation" in Matthew 24:34 incorporates the elements of both race and time. Ethnic Jews (the race element) living when Jesus prophesied, were destroyed within a generation (the time element) in 70 AD.
There's room for both.
This is not really the issue with the OP though. The issue is how to use a lexicon properly.
Many on this website will appeal to thayers to say “Genea can mean family of same stock”. This generic statement is true. Thayer’s definition number 2 states “ family of same stock”.
The problem is the next step. Thayer’s lexicon is a tool that lists the broad meaning of words AND THEN lists verses under each meaning demonstrating how the word is used in a specific context.
So for example, thayer’s provides the meaning of “family of same stock” and then lists verses under this definition as to where you will find this meaning in the NT. However, Matthew 24:34 is NOT found under this definition. Therefore thayer’s has not designated it to mean “family of same stock”. Instead Thayer listed Matthew 24:34 under “whole multitude living at the same time”. That is how Thayer’s defined it in the context of Matthew 24:34.
This is not to say that Thayer’s has the absolute authority as to what genea means. This is only to say that if you are going appeal to thayer’s, then use it properly. It’s absurd to appeal to thayer’s, and then completely disagree with it, cherry picking it, and committing the fallacy of illegitimate totality transfer.
What would be extremely useful, is if someone could provide another lexicon that defines genea, in the context of Matthew 24:34, as family of same stock, instead of abusing thayers.
