Open systems are not a guarantee for commercial success.
Look at M43. Open, but hardly the best selling mirrorless system out there.
While I understand that sometimes customers will get frustrated at the closed nature of some systems, a certain amount of lock in is necessary for long term success in most business models.
It depends what you make, what you are good at making and where your position in the market is. Sony is an electronics company without much of a background in making lenses (hence the reliance on Carl Zeiss) and they're starting from a position where no-one has Sony lenses and most people are 'locked in' to either Canon-mount or Nikon-mount lenses. If they want to sell bodies to professionals who use multiple lenses (beyond those who are simply using them as digital backs and manual-focusing everything) they will need to let them use their existing lenses at full capability. If they don't do it, then Panasonic, Samsung or someone else will, once they have the technology to do so.
It's how it works in the large format and MFDB world. You can stick pretty much any large-format lens in front of any large format technical back or medium-format digital back, so long as the image circle is wide enough to cover your sensor. You may have to adjust a few things (e.g. mount board) but it still works normally.
Besides, there are still accessories such as flashes, batteries, etc. Not as strong a lock-in as a suitcase full of lenses, but still something.