90% would probably never know, if the manufacturers do their job right. Slap "no user serviceable parts inside", "Warranty void if removed" etc. stickers everywhere and don't allow testers who get samples to show the inside, have them sign a non disclosure agreement.
And apart from that, they can have custom input sections with extra input options, custom power supplies, nicer cases, additional features, customer support and certainly a higher resell value. A good product stays a good product in my opinion, regardless if parts of it are available individually. And let's be honest, for someone expecting a complete, working product without any hassle the DIY route is often times not satisfactory or even an option.
Edit: Obviously a "good" product stays a "good" product. That "good" was less about the measureable performance, but more about the price/performance ratio. If that was perceived as good compared to the non-DIY competition before, it will still be even if parts are available for DIY. This way my statement should make more sense.