My take is that the main reason the T-coil never took off, and might now be on its deathbed, is because (as a general rule) audiologists have never pushed them. Every time I wanted to add it into an aid over the years I was told (by different audis) that I didn’t want it as it would just add to the expense. It was always a fight to get a T-coil in one, let alone both.
Having said that, to go further and say that a T-coil comes close to the quality and performance of Bluetooth is just not honest. Just as Sparafucile stated, in the modern world of today T-coils are almost impossible to use in most offices and even in cars (my car’s computer can be heard louder than my cell phone when my phone is right next to the aid! Fluorescent lights, phone poles, grocery store electric doors, etc, etc, etc all wreak havoc when trying to use T-coils.
Recently I purchased a set of Alera 9s (with wireless & Bluetooth capabilities) and to be honest, was very skeptical about getting aids without a T-coil! When I purchased the aids the newer Alera T-coil models weren’t available. However, in retrospect, that was probably a good thing because I would have purchased a T-coil equipped older style (non-RIE) much larger BTE aid and, as it turned out, almost never (I’m trying to be generous) use the T-coil! The quality of the wireless link, the ability to understand what is being said because the signal to noise ratio is a million times better than the T-coil, and the ability to use it just about anywhere regardless of which electrical accessories are in use around me means that, for me, there’s not even a decision to be made. As stated, I’m someone who grew up with, and fought for, T-coils my whole life, but if I’m forced to choose between wireless or a T-coil, for me there is no decision to be made! Wireless will win hands down every time!
Lastly, in an attempt to present this as honestly as possible, it’s only fair to admit that wireless aids are not perfect and do have their own drawbacks. For instance, they (1) increase the cost significantly, (2) eat up batteries like they are going out of style (if the streamers are used often), and (3) are indeed 2.4Ghz transmitters/receivers placed directly against your head.