Well, again, we’re not talking about a real court of law where this is going to matter. It will be in a court of quasi-public opinion in an impeachment trial setting or other pressure from the GOP if they see writing on the wall a la Nixon (admittedly a very unlikely scenario right now).
As for Comey not “being the investigator,” then according to your logic, if Trump had said to Comey “stop investigating Russia or I’ll fire you” there would be no obstruction of justice, either, because Comey is inconsequential to the investigation. Again, I’m not a legal expert, so maybe that technically isn’t obstruction, but I’m confident it would be enough to remove him from office in an impeachment. What he’s done so far doesn’t rise to that level, but it’s hard to believe that we’re at the end now, when Trump and his team have been saying that for 6 months and more stuff keeps coming out. But who knows.
As for “Trump did have something to worry about,” you’re confusing his actual motives, which are truly hard to discern (if he cared about keeping focus on his agenda, maybe he should stop tweeting wildly off-message distractions), with motives as they’d be constructed in the trial.
Think of it this way. Imagine a world where it’s 2019, the Dems have taken the House in 2018. BUT, all we know about Trump and Russia is what we knew as of July 1, 2017. So we’ve got 18 more months of investigating with nada. In that case, the Dems have a very weak impeachment case, politically. It’s very easy for Trump and GOP Senators to say “he had nothing to hide, so why would he have been worried about Comey finding anything.” Maybe they Dems force the impeachment through on partisan grounds, but they know its dead in the water and may not end up being a useful cudgel against Trump. So there’s a greater chance they don’t bother.
That calculus has substantially changed now. They go from having no real case to a case that people like you, who seem to know a lot more about interpretations of obstruction of justice law than I do, have to explain in detail to me because it only makes sense on a very technical level. This is bad. In many ways its oddly equivalent to the Hillary Clinton deleted e-mails issue, where it would be hard to prove that Clinton technically broke the law by deleting those e-mails, since as I understand it this would involve proving an intent to obstruct, but it is obvious to anyone that she did something that is not right.