Interesting question because Sony had a monopoly in the last 2 generations and they were brilliant generations in terms of gaming for me - and it didnt really decrease the quality of the games from the competition too drastically in my opinion.
I dont really care who has a monopoly or whether there is one, as long as the quality games keep coming, and if MS continues the way they have been then its great for me. MS is a software/PC company through and through and they have transferred their values into the games market. So even though they have 70% share in total game industry revenue, I dont think they will rest in their laurels in regards to software/online ventures.
Sony just had a hiccup when they took their eye off the main thing - the games - in their initial vision for the PS3.
I doubt any company will have a monopoly this generation though as the industry/consumers are growing rapidly.