If there were ONE and only ONE, programming adapter for ALL field-programmable devices, would it matter at all what's going on?
YES!!! some implementations would have to be so convoluted that throughput would suffer greatly and being so convoluted would likely be code hungry an error prone. What is good for the goose is not always good for the gander.

As to customer updates I can not see customers (with very few exceptions) of small embedded systems having the willingness or knowledge to attach a different interface for program updates. At a previous place of employment I found some customers to be - computerwise - 40% less intelligent than a hole in the ground. Thus if your thingy is attached to a PC via RS232, that is how you must do it, if you have a USB slot and no umbilical cord that is how you must do it.

It may be that your customers are very smart, if so, I congratulate you, I, however can not require computer litteracy from my customers.

Erik