I can see Directv's argument, they don't want to support people's home networks. And they shouldn't. But I don't see why people in these forums keep advocating DECA to people who ask. DECA is an abomination, created to solve a specific problem of people have coax available in locations but not cat5. If it was common for cat5 to be run to the same locations coax was, DECA would have never been invented.
Ethernet is far more reliable than anything using coax - there's a reason why Ethernet abandoned running over coax well over two decades ago, switching to twisted pair. The argument that it "gets the traffic off your home network" is silly, given the fact that an ethernet switch only forwards traffic to the port it is destined for. So you can be transferring big files or playing games over the same network you have multiple RVU sessions and you'll be just fine because the traffic is independent of each other. If you're worried about it, gigabit ethernet switches cost almost nothing these days, and make DECA's bandwidth look like dialup by comparison.
Running over coax just makes more sense for Directv and their installers because most problems that would cause issues with DECA would already be causing other issues Directv would need to fix anyway. I don't know about everyone else, but I don't do don't things inconvenient for me to make things convenient for those I'm paying to provide a service.
Ethernet moved from coax to twisted pair primarily because of cost...most office buildings had plenty of available twisted pair, but no coax. Plus, it was easier (i.e. cheaper) to wire a star topology than a bus. It had almost nothing to do with reliability. Thick coax Ethernet was as reliable as any other topology before or since. Thin-net was susceptible to the bus being broken if someone disconnected a t-connector, but was otherwise reliable.
Otherwise, I agree with your sentiment. Personally, I treat our network as a one single network with some cat6 and some coax segments. At the moment, all the DirecTV receivers are attached to the coax, but so is a Roku and a BluRay player (there was a spare coax in family room, but no cat6). I also ran whole home over Ethernet for a couple of years, and up until I moved things around after we got a Genie there was still one DVR on Ethernet.
Just one point about throughput...what you say is correct, IF the network is designed correctly. But I have seen lots of people string switches together in such a way that lots of traffic passes over a single wire. For example, if someone is streaming a film from Netflix, another person is downloading some software, and another is backing up their computer, and all of them are attached to the same switch as a C41, and the Genie and the backup drive are both attached to the router directly, then you'll definitely have some bandwidth issues when watching TV via the client if you have a 100Mbit connection between the router and the switch.