<They would never have opposed the appointment of a new FBI director>
That's very naive, at best; but since it's also speculative (we can not know what might have been), I won't belabor it.
<any investigation would have proceded exactly the same with or without Comey, or with a new Director or acting Director.>
Even if that's true (I think it is, more or less), that's not the point I made: the point was that had Obama fired Comey, Trump and the Republicans would have accused him of trying to influence the election. I think Obama can rightly be criticized for being overly sensitive to that.
Firing the Director does not stop the investigation; by firing Comey, Trump probaby gave the investigation a boost. But Trump has been clear that he's trying to impede it; he's just too dumb to know that he can't, not in any significant or lasting way. (The White House is not Trump Tower.) I don't know whether his bumbling attempts to interfere rise to the level of obstruction,  but we'll find out.