That disposable mag rumour comes back around all the time, with no truth behind it. They were not significantly less expensive either. Having been around for the entire life cycle I have first hand knowledge of the entire program.
The truth is that the original CF Thermold mags were made out of zytel with no glass fibre reinforcement per CF request. Later versions were improved by thickening the lips and injecting glass fibre into the molds. Mags with date stamps after 95 are actually quite good and surpassed the aluminum mags in nearly every durability and reliability test, but it was too late, and once troops lost confidence there was no going back. Many overseas Diemaco users, like the Danes and the Dutch maintained the Thermold mags well into to the 2000s with very good result.
The later versions had thick machined lips, a 95 or later date stamp and improved follower if you knew what to look for.
The nice thing about them was they were impossible to dent without destroying them in a very obvious way, same goes for cracked feed lips. Anyone who took a hammer to one to decommission them knows what I mean. The aluminum mags are easy to dent and it's hard to tell, and the lips are easy to bend and its hard to tell. They also had a huge reinforcing lip that made them very resistant to damage when dropped mag down. Aluminum mags take drop hits all on the hole for the latch and the lips. Spot welds also tend to fail. I did lots of side by side testing, and I am not a fan of the aluminum USGI style.
The down side of the Thermolds is they are not stable at very high temps - like summer in Afghanistan in the back of a carrier hot. This is why the Canadian ones are not made any more. If you get a hold of one - especially with our neutered mags - it would likely last you a life time. I shot four NSCC and the whole seasons leading up to them with later version Thermolds - never had a crack or a stoppage.
I've since moved on to lancer and magpul gen III, but the later versions were actually a lot better than most people give them credit for.