It's not so much about market share as it is profits. What's the point in securing 80% market share if it costs you $20 billion to do that? There's not a company out there that wouldn't pick 30% market share and $1 billion a year profits over 70% market share and be somewhere around zero net gain. The interest in high market share is the potential to monetise that and generate profits, but that's not the end game. If MS can switch XB1 interest to mainstream consumers and sell 25 million to high-sellers buying content on MS platforms (MS movies, MS music, MS games, yada yada), XB1 will have been a good venture for them.
The problem with discussing the console market is those engaged in the conversation typically discuss it regards the 'console war' perspective where there's only one winner. Whatever PR that businesses may present, it's all about making money, nothing else. 'Best selling console' is only a PR point.