What exactly do you think they are doing now? They just acquired Turtle Rock, all their games have been 1st party, most of which began as mod or third party development ideas and Valve bought/hired the staff/IP. So I'm sure Vale will thank you for suggesting to do exactly what they are already doing!
In my opinion they are flirting with this direction and have not committed to it. They do offer many games through steam which is great but they have not gotten exclusive publishing rights for any large titles.
I'd like them to change that and start picking up studios the same way the other publishers do in order to attract more customers and bring in more revenue.
This is a choice that many business owners will face and it boils down to money. Do you want to continue risking the potential value of your company or sell what you have now and let someone else risk their money?
The 2nd option involves less risk and less potential return in the future. The 1st option involves more risk and more portential reward in the future.
The movie, "There Will Be Blood", had an excellent scene where that guy had to make a choice, sell his land to the oil company and make millions (a ton of money at that time) or do it all himself and make hundreds of millions.
So many companies today choose to just get out and let someone else get all the money and that is a dead end street for the types of games I love. You see what EA and such does to the studios it picks up. There is no telling what will happen if Valve expands, they may fold and go under and never be seen again, they may grow into another EA, or they may grow into a great computer game company/publisher that we've never seen.
But in business if you're not growing you're business, you're going out of business. Either keep growing or close up shop because it never stays the same. You have always got to adapt and keep moving. Take new risks, go after new customers. Staying with the same old stuff is why companies fail.