I can watch Roku/Netflix on my elderly TV via an RF Modulator. However, when the Roku is connected to the RFM I can no longer receive cable on the TV. If I change channels I just get noise. This is a single channel RFM.
That's pretty much how most low end RF modulators work, not matter what video source you are using them with. While they are a simple/cheap way to hookup video sources (ROKU box, video games, DVD players, etc) to an older TV, they also cause interference with many of the other "channels" on the TV at the same time. So the problem isn't the NetFlix Player per se, its the technology you are using to hook it up to your older TV.
There are many solutions to this problem, depending upon how fancy you want to get (and how much money you want to spend). However, IMHO the cheapest/easiest solution is to just get a manual video switch, and hook both your antenna (or cable) and your modulator box up to that switch. Then you can just flip the switch for which you want to watch. Since TV (video) switches of this sort start at under $10 (and even the ones that handle more than 2 video sources often can be had for under $30), this is a cheap and simple solution to implement. And this solution does save you having to swap cables around all the time. However, such a solution does have one (minor) drawback, because those cheap video switches are mechanical switches. So unless you want to spend a lot more for a video switch with a remote control, you will be stuck with walking up to the TV (to flip the video switch) whenever you want to change between the ROKU box and you other video source(s).
Of course, (if you have the money) you could also consider upgrading your TV to one that has built-in support for directly plugging into the ROKU. For example, a year or three back, I was able to buy a 20" standard definition stereo TV (i.e. not HD, but still a decent normal TV for the price, including stereo sound), for under $150 at Walmart (and such TVs are still available for similar prices, or even cheaper, if you look around). And that TV not only had direct AV inputs, it actually had two sets of them (on in the front and one in the back), in addition to the normal "antenna" input. So I'm able to directly hook up my cable TV (via the "antenna" input) and my DVD player (via one of the TV's video inputs) and my ROKU box (via the other set of AV inputs on the TV). And since this is all directly hooked up to my TV, I can easily choose between the video sources (cable TV, DVD, or ROKU) via the TV's remote (no "interference" at all, nor any need to mess with a "modulator"). Of course, there is nothing preventing you from buying a fancy HDTV if you like, and a newer HDTV will help you be prepared for the newer digital TV that is coming, I'm just pointing out that it's not necessary to spend that much money on a TV just to get video inputs (as cheaper TVs are still on the market).
And many even fancier options are available for a price. For example, there are fancy remote controlled video switches that you can get (so that you could choose your video source from a universal remote control), but they aren't nearly as cheap as the simple mechanical video switches. And it's even possible with some fancy (and reasonably expensive) modulator equipment, to give yourself a special high numbered "cable channels" on your TV while also having the normal cable channels work (so that everything can just be tuned in with the TV remote). But such solutions (while fancy/elegant) are moderately costly (many of the fancier modulators cost more than a low end TV, for example). So unless you really have the money to spend (in which case why not just get a newer HDTV with multiple hookups and be done with it?), going with a simple mechanical video switch (or buying a new decent standard def TV with multiple video inputs), is probably your best bet.