How many teams do you think would be up for a stand-alone championship game this year? The top 3, that's it. Alabama, Notre Dame, Clemson. Don't even start with UCF.
Go ahead and show me a year where there were 5 deserving teams. I'm going to guess if that's the case, the #1 is far above the other 4.
The problem is, deciding who "deserves" to be in is opinion based.
Washington won the PAC-12. Because the PAC is considered a "weak conference" or having a "down year" and Washington lost 3 games, many have the opinion that they don't deserve to be in the playoff.
However, there are also many who have the opinion that because they are a conference champion, they should be in. In an 8 team playoff they would be.
Additionally, so what if teams get in that some feel don't "deserve it"? If they truly don't deserve to be there, it will be proven on the football field by players rather than in a board room by people in suits.
And why not "start with UCF"? They have been unbeaten for 2 straight years and beat #7 ranked Auburn in the Peach Bowl last year.
Finally, an 8 team playoff means more football and more football is a good thing.