Show me an audio amp with a brick wall filter that will clean off everything beyond the Shannon rate of an incoming 44.1ksps signal. As for re-digitizing a signal, what happens when you re-digitize an unfiltered 44.1ksps signal at 96ksps or 192ksps? Those digitizers won't filter away the mess below about 48kHz or 96kHz.
I expect the reason you don't see this filtering as important is you haven't experienced what happens when it isn't there. Most audio DACs have had the filtering integrated since the mid 80s. in early CD players they used to up sample by 4x or 8x the 44.1ksps rate during the ZOH pre-compensation, just to make the analogue post filter simpler. Now most DACs are sigma-delta, the filtering requirements are must easier to meet, but still important.
As you stated, most, if not all modern audio DACs are sigma delta, - their output is updated at 64/128x the sampling rate, pushing the sample rate waaay up the spectrum, meaning the a simple output filter, combined with the frequency response of an amp greatly attenuates it. I won't go into interpolation/digital filters etc.. as ultimately it just means that at the output, the sample frequency is now in the MHz range, and is already attenuated. Unless one is plugging the output of their DAC into an RF amp, I don't see why output filtering is particularly important. Yes, it is needed, but I can't see it being something that requires a great deal of design?
The conventional (say, R2R ladder DACs) that really do require strong attenuating filters are, these days, the ones designed and built by 'audiophiles' because they believe that sigma delta DACs sound 'sterile' and because they've seen 1-bit SD modulation waveform and thought it doesn't 'look' like nice clean audio. The irony being that in an effort to bash modern audio converters, they've resorted to older technology that performs measurably worse, but 'looks nicer' on a scope (and also costs a lot more because of very tight component tolerances, which justifies the high cost for those silly enough to pay for them).
Some sources of that misunderstand the process about it:
http://www.mother-of-tone.com/conversion.htmhttp://www.msbtechnology.com/faq/why-ladder-dacs/https://hifiduino.wordpress.com/2014/10/12/r2r-for-the-rest-of-us/Note the last one isn't snake oil or anything, I'm sure its a very well designed R2R DAC, but seems like a LOT of effort and money for something that performs very similarly to a $2 IC.