For better or worse, it IS the recording medium. Clever marketing and a timely arrival as a full fledged tool in the 90's seemed to secure its place. Back in the day, the TDM architecture allowed any slow ass computer to actually process audio (albeit poorly) without dragging the very limited system preformance down. Even with today's faster computers, having those cards do some of the work still seems appealing. It also adds to the platform's scalability. You can add cards up the wazoo to really obtain some sick amount of DSP. You can also add multiple interfaces to get a large number of I/O. Now, it's not that you can't do this with other systems nowadays, it's just that PT seemed to get into the market first with a very USABLE this form technology and has been there since.
Digidesign's foray into the lower end market with it's PTLE products was also one of the more brilliant steps towards total market domination. Having a compatability between the low end and high end systems is a big step for middle of the road users. That middle market where people can work on lower end systems for many projects (say, tracking) but then go to a higher end system for mixing, etc. Very big move.
Digi's virtual monopoly on the DAW market has just simply built on itself. Try to think of a pro studio that doesen't at least have one rig sitting around. For every one that doesen't, there's a thousand that do. Back in the day when cutting to 2" 24-track, you would go to other studios and simply expect to be able to throw up your reels and got to work. Granted some had Studers, some had Otaris, etc. , but you could work. Try going into a random studio nowadays with a Nuendo session.......You're up **** creek......But, you go in with a PT session and odds are that you can work.......
As for sound quality, it used to be terrible. But, it HAS definitely gotten better. It's all in how you use the tool. If you track everything blazing hot on 888/16s without an external clock and mix ITB with loads of cracked plugins on everything, odds are that unless you're Elliot Scheiner (sp?), it's gonna sound like poo on a stick (I think Fletcher said that one!). BUT, if you get an HD rig, put a Cranesong/Apogee front AND back end on it, clock your interfaces properly, track proper levels at higher sample rates, minimize your use of plugins, use a good analog console with some choice outbaord to mix on, then one DEFINITELY has the capability of making a fabulous sounding recording.
PT is simply a tool with strengths and limitations, it's all in how you use it. It reminds me of a certain brand of console that seems to have made a similar domination of the market a few years earlier. Is an SSL 4k the best sounding console out there? Not even close by most peoples' opinion. Have people figured out usable tricks/modifications to minimize its weaknesses and emphasize its strengths? Absolutely! It's a matter of learning the tool and adapting new ways to make it sound good. It always has been.
As for other DAWs, the day I start getting asked to work in Nuendo/Logic/Cubase/DP on real projects for real money (not John Q Notalent's $200 demo) then I will seriously consider these other platforms. However, since that has NEVER happened, I'll stick to PT. It pays the rent and I'm OK with that thumbsup