Why can't it just be unlocked and the consumer can decide if they want to use Prime Mode or not? It's no difference than UAPP on an Android phone. I can use any app as it comes or go through UAPP. Characterizing this as a "Prime mode" or the highway is a terrible attitude to have to your customers.
Okey but why can't you offer it both ways? Let the people who want to use Prime Mode use it. But also let the other people who don't want to use Prime Mode because they have to use your Music App the option to use a different Launcher and thus disable Prime Mode? The more options that are available to potential buyers the greater chance of selling more units which I am sure is your main goal.
Buyers want more options for how to use their DAP's not less. Nothing turns a customer off faster than telling them a product can't do what they want it to do. The people on this forum are telling you what they want but you are content to tell them what they can't have. Not a recipe for success based on my experience selling consumer electronics.
I'm going to play devils advocate here and give some answers based on my experience as a product designer and some good ol' fashioned guess work.
Disclaimer: I'm not affiliated with Shanling or any other audio brand but will say I'm interested in both the M3 Ultra and M6 Ultra as potential additions to my collection.
I haven't used the Shanling music app yet so I can't comment one how good or bad it is. But using a different music app as your main player app is something I can understand, I have a device whose music app is downright horrible. That said, expecting for a manufacturer to make exclusive features like Prime mode available for apps from the competition is a downright unreasonable expectation to have from any brand. No matter how much you want it. It's not like you can't use other music player apps at all, it's just a extra feature for Shanling's own music app.
Aside from ethics, something like using a different app in prime mode is a lot harder to implement than you think. Getting all the processes in the background to work correctly as well as keeping up with the updates of the other apps to make sure of compatibility take a lot of time and work, and time costs money. No company is going to spend money to make exclusive features available to their competition. It makes no sense from a business perspective.
And the UAPP example is not the same thing, that's simple routing of a audio signal which can be done without controlling another apps processes.
You don't engineer for the minority, given that said minority is under 20% of (potential) users. If it's barely a minority, say around 35%~40%, some action should be taken. Some exceptions apply if it is something that is easy to add and won't take much time or if the missing feature is common among the competition and the effort is deemed to be worth the payoff, e.g. gapless playback. I personally don't use gapless playback much if at all, but do agree it should be there as an option.
A exception on the opposite side of this is that if the cost is too expensive or it's too complicated to be feasible even though a majority wants it, it should then be explained that it's not possible for X or Y reason.
Now if this was a open source project and the original creator made a function like a exclusive mode only for his favorite music app, you'd have a point. But it's not a open source project. Shanling has probably sold well in excess of 1000 DAPs that have this Prime mode feature and there are less than 10 people asking about this particular use case. It makes no sense to spend any time on this from a financial point of view or from a customer service point of view. Losing less than 0.1% of (potential) customers isn't going to make any difference to any company, and the costs behind trying to get that last 0.1% is simply not worth it. It's not like any other manufacturer is doing this either that I know of. You can't make everyone happy.
In my personal opinion, if you want to use a different music player app as your main app or even want change the launcher/OS, that is fine and the device allows this. But it's not the manufacturers job to make sure everything works perfectly when you don't use the device as intended (different OS/Launcher or loss exclusive features with different apps). That's all on you as the consumer as it's your choice to use it in this way.