Okay,
Firmware Updates come directly from MS for Lumia Phones. It's the OS Upgrades/GDR/Builds that the carriers delay.
The scuttlebutt is that MS is going to bypass the carrier with Windows 10, however, after the initial release of the information, I have really not heard of anything really substantial on this.
As others have stated, if you want the RTM release (final) before the carriers decide to do whatever it is they are going to do, and MS is not bypassing them, stay in the Insider Program as it will be the only way to get it until the carriers release it.
With this being a HUGE (as in marketing) OS update for mobile phones, carriers should not delay too much on the certification and roll out. However, those with Flagship level devices will see it first and then going down the line to the low-end devices. Some may never see the update from the carrier and will only get it through the Windows insider Program.
Hope this helps clear things up.