I have to agree with @FlyingDiver here. Z-Wave has been on the market since 1999. It supports up to 230+ devices on a single network. There are probably 30+ companies making Z-Wave devices, and there are literally thousands of devices on the market. Reliability is
significantly better than the average X10 installation, though there can be range related issues (usually solved with strategic repeater replacement). Reliability of the hardware is just like any other hardware - some vendors make higher quality devices than others, so going the lowest cost might not get you the same quality as spending a bit more, though I've yet to see any significant proof of that.
The Z-Wave protocol is, frankly, light years better than X10. Each message is acknowledged, if the ack isn't received it's retransmitted (all done at the protocol level automatically). Most mains powered devices also repeat the signals making for a much more robust mesh network. The same can be said of Insteon, but it still can suffer from power line signal issues and it suffers from being a single-vendor solution.
Your installation design, while it might have been good to help reduce X10 signal issues, is more complex as Joe points out, because it requires home run wiring. Standard wiring is all that's needed for everything else.
His arguments are pretty weak:
(1) unable or not designed to manage so many devices
As I said, one Z-Wave interface can support well over 200 devices. Other WiFi based systems are unlimited, though they are newer and thus don't have the track record that Z-Wave or Lutron has.
(2) there was no reason to believe for how long any of those protocol would be in the market
This is just silly. There are millions of people using Z-Wave products (even if they may not know it). It's been around for 20+ years, is being actively developed by Silicon Labs which is very stable and reputable technology company that's been around over 20 years.
I would recommend doing your own research, as your X10 guy may have other/different motivations than you do.
Note: I'm not recommending that you switch to anything, but rather just pointing out that you have options that are likely better if you want to consider a transition.