No. Not talking with the police in the first place would be the right choice so I'm not claiming either way is wrong.
As mentioned, if he immediately plead the 5th he would have been labelled as suspicious and guilty by public view. Let's assume he is innocent, and of course it is you being questioned by the police. (which is the foundation for our justice system)
Cop says "Hey, can we chat?". You being innocent have nothing to hide. "Sure, what's up? Selling fundraiser stuff?" Cop says "Nah, just making the rounds. Chief says we need to interact with the community more. I think he's right, even though he can be a prick at times. You know how bosses are." and nudges you with his elbow to let you know he's joking. You return "Yeah, my boss and every other boss I ever had gets on my nerves sometimes. So what kind of community outreach do you have planned?". Cop returns "Well, we were wondering who had interest in guns and maybe putting on some gun safety classes and shooting range classes for the public.". You "Really? I have a couple shotguns handed down to me when my gramps passed away. Never used them, but it may be fun to try.". Cop "Really, we just had a murder with a shotgun. What kind of shells do you own?" You.. "um, can I get a lawyer?"
Given the above scenario, would it be fair for the cops to haul you in front of a jury and begin telling them how as soon as he asked about ammo you asked for an attorney and refused to answer his question? Absolutely not.
I'm not saying that other evidence did not end up making the difference in conviction. What I'm saying is that the tactic described above should not happen. You became uncomfortable with the officers questions, and decided it best to stop talking. That fact should not be admissible to a jury, as it requires speculation on "why" you plead the 5th and started having concerns with your rights. Also, don't compare this to other shitty things done to get convictions. As they say, two wrongs don't make a right. But we as a people must start standing up when we see obvious wrongs.
To your last paragraph, I agree with what you summarized. That is now how we are supposed to work though. People tend to see no issues with a double set of laws (one for them and one for us) until it's them on the receiving end of the unfavorable rules. That double standard is what we have to start trying to change. Complacency and apathy have never fixed anything.