All the current ND streamers have a coaxial digital output that enables them to bypass the internal DAC.
I think the reason ND5 owners go down the nDAC route is that it’s cheaper than replacing it with an NDX2 and in doing so they can convince themselves they’ve got a cheap “upgrade” to the ND5, there’s also the “I don’t need a display” argument which as I’m sure we all realise has nothing to do with the performance of the streamer, if it did I’m sure the ND555 would be screenless.
There’s also the reasoning that because you can connect these things together it’s a good idea, I’m sure if it was such a good idea Naim would make a standalone DAC and outline it as a suitable upgrade path to the ND5 but they don’t do either.
I expect you’ll find more folks on here that use a non Naim DAC with an ND transport to alter the sound presentation from the one Naim offer. A Chord DAC being an obvious example. I setup my Chord Hugo 2 like that for a while, sounded ok, but added a load of extra cables and mess I prefered to avoid so it was merely a curiosity.
I’d be inclined to loan something like a Chord Qutest to get a feel of what can be changed, a nDAC is primarily appealing based on it’s used cost, I’d certainly not limit my choices to a discontinued Naim DAC alone.