Where is the reasonable suspicion that the law had been broken by a Biden? The story with Biden was that he was getting paid too much money to be a board member in a field he has no expertise in. That isn't a crime, its poor management.
It's the same as with the original inquiry - influential people magically receiving money for not doing anything is rarely "poor management" (though we can't discount that, and it will remain the default position until the new investigation is concluded). It's suspected (as is often the case) that bribery or some other form of corruption was at play. These things aren't normally just shrugged off - they get investigated. This case should be no different.
[Trump was asking to investigate the Bidens and not the company]
This strikes me as a very forced distinction. Investigating one strictly requires investigating the other. But perhaps I'm being too generous.
[Biden was justified in pushing for the dismissal of a prosecutor who just coincidentally happened to be investigating his son]
Perhaps. However, the prosecutor being dismissed doesn't necessitate the
investigation being dismissed - and the fact that it may have been dismissed improperly was acknowledged
before Trump got involved.
If this request were made in an official and straightforward manner, then you would have a case. But it wasn't so you don't.
They had a phone conversation, and then the conspiracy theorists started spasming again. Neither of us knows whether a formal request will follow, because the proceedings are still ongoing.