Components on floor

Wall mounted electric fireplace? What is that... does it have motors and re-stats? It might create some interference...

I agree that isolation would be necessary. Springs at least. The floor is seismically active and will transmit the truck and car traffic, actual seismic activity. I have a seismometer in my house. It is surprising the difference in background vibration between day and night. This is why Townshend Seismic platforms (under speakers) are so effective... but also important for components. 

 Is this a bad idea, and if so, why?

Yes.  You have to bend over to use them.  Must you put them there?

Do you mean that the hardwood is directly attached to the concrete (ie: glued?)  My components sit on the concrete slab under my house, si the result should be similar. A 10,000# slab is pretty stable so there's no problems with vibration, though I  understand that surface waves can travel on top of the concrete but that's obsessive, even by my standards. Wire dressing is no more of an issues than with a stand. Bending down at my age IS an issue. The preamp and streamer are both remote equipped but the turntable is a pain! I've considered a sand filled concrete pylon to put the table at chest height but have not yet done so. Dust needs vacuuming daily. A unexpectedly significant improvement comes from not having anything sound reflective behind and between the speakers, which is where my components sit. It's like gaining an extra foot of wall to speaker distance. Overall, I love it!

You should be fine.  I have a floating wood floor and I have a lot of issues with vibration.  We have an extra bedroom in the basement with a cement floor and there’s absolutely no issues.  

I have mine on finished plywood which sit on short legs. Each component sits on anti-vibration pads. Only issue is the dog likes to lean on the plywood!