Think of it like this. Your goal is to run 17 flat or just sneak under. You run 17:50. Nobody in the history of running failed like that, due to what they lacked being speedwork. Their limiter was their aerobic capacity is not yet up to scratch and should have worked on that, rather then killing themselves doing 12x400 or whatever things I see fellow 5k enthusiasts doing at my local track. That is a very basic example, but essentially the simplistic version of what is going on here.
It's much easier to push your ability up from below, until you run out of room to do so. Spoiler, I doubt most in this thread run enough to get to that point. The other as advantage is you are basically ready to just hop in a 5k-HM at literally any point.
If you are in this thread, you are a hobby jogger. We are in races that are just essentially glorified time trials. Everyone is for the most part doing their own thing. If you pace it correctly when training like this, the absolute last thing going through my head in the last 400m is me thinking in a 5k "oh wow I wish I had some of that top end speed right now".
Unless you are racing at a high level or really care about just 5ks, you are probably only giving up a handful of seconds at worst training like this. If you also consider the fact everyone is dying at the end of a race, this will probably counter that by keeping your strong and dying the least by the end.
There's a guy I race against sometimes. He has ridiculous raw speed naturally, or whatever you want to call it. He sat on me in a half once and I'm not joking, basically spirited off like he was usian bolt compared to me in the final stretch. Flip it my recent 10k I won. I am one of the slowest speed guys on the planet. But with not long to go, I pulled away from him with a tiny increase in pace. If he didn't know me, he would think he got out kicked by a monster. All that happened is he slowed down more and I sped up slightly - I was just aerobically stronger to maintain pace. That's basically all that is happening at the end of a race at this level, anyway.
A bit of a tangent, but I get questions like this a lot with people messaging me, scared, saying "BuT WhAt AbOuT SpEeD?!"