If both Sony and other third party manufacturers supply adapters, for the multi-interface shoes that allow non-Sony accessories, how would this be forcing anyone to buy only Sony equipment? If anything, the low cost adapter from Sony and elsewhere makes it easier to use third party or Sony-proprietary gear - the choice is up to the end-user.
Other than the multi-shoe adapter, the HDR-PJ540 uses USB connectivity, MicroSD or Memory Stick micro cards, standard micro HDMI, standard wifi, standard 3.5mm stereo audio input, can interact with Android or iOS devices for various remote capabilities and captures standard AVCHD-compressed video that can be edited by many non-Sony products. Nothing in there that requires Sony-proprietary products.
The built-in multi shoe is a hot shoe that allows connectivity for lights and mics if the user wants "ease of use" accessory power and connectivity and decides the Sony proprietary stuff is OK. The available optional adapter allows third party gear, just like any other cold shoe - such as the one provided by your HC-750's (not built-in) shoe adapter. A camera bracket with cold accessory shoes would provide similar functionality.
There may have been a time when there were issues with Sony forcing proprietary gear, but many manufacturers do something like that... even Panasonic (at the higher end: LANC connectors are Panny specific, use of P2 cards, use of DVCPro/HD format, a few others). And certainly car manufacturers don't all have interchangeable parts, but not being able to easily put a Lotus bumper on a McLaren is not a good reason to not consider purchasing a Lotus. Not being able to use a Galaxy S4 case is not a good reason to not consider an iPhone.
And yes, I've used Panasonic camcorders (PV-GS320, AG-HVX200, others). I currently use Sony camcorders with the same Audio Technica, Sennheiser and Shure mics; Lite Panel and VidPro video lights and various steadying devices - some of which were used with the Panasonic cams... and for the remaining Sony miniDV tape based camcorder, I use a Panasonic tape head cleaner. You current assumption of "Sony proprietary everything" might not be entirely accurate. But I agree that is the end-user wants a mic or light compatible with the multi use shoe for power and connectivity then they may have limited third-party options.
Both Panasonic and Sony make fine camcorders. Sometimes increasing their functionality means adding more equipment - sometimes it is proprietary, most times it is not. In this case, availability of an inexpensive adapter or use of a third party accessory shoe camera bracket easily and inexpensively deals with the (perceived) proprietary issue - but you are certainly welcome to your opinion...
Since the original poster did not tell us what they are planning to use with the accessory shoe, we don't know what the requirements are and we don't know if Sony-proprietary or third party equipment should be considered/suggested.