What absolute bobbins. Are you suggesting there's no difference between playing 3 group games and 1 knockout game ? You could fly halfway round the world, get royally shafted by the ref or VARS or chronic bad luck in your one and only game at the tournament, and be on the next plane home. And thats that for another 4 years. Terrible idea.
The need for three matches is to make it worthwhile for fans to travel to the matches and get tourist money into the hosting countries. The fans would love seeing elimination matches much more than the 3/4 dull match that England and Belgium played until it suddenly changed because the national side forgot it had to get a clean sheet if they weren't going to score in this match.
What is so much better about getting 2 dull matches and one elimination match?