As mentioned before, HS tennis is not a stepping stone into the pros as with college. HS is supposed to provide student athletes with a chance to participate in the game, grow and learn. Many can not even play at a 4.0 level so how can you expect them to play college format tennis?
I understand what you're saying about the top kids on varsity but the point is, there really isn't that big of a talent pool. I live in NJ where basically only 1st and 2nd singles is competitive on varsity due to the number of nationally ranked players playing those positions. Here's is an example, I train at Garden State Tennis Academy where they've trained numerous sectional, national and state champions including a Junior USO Quarter-finalist. They all played first or second singles at their highschool.
Jonathan Carcione: 2011 and 2012 State Champion. 1st singles at Ramapo. #4 Boys 18s ETA. Plays for Princeton University as of Fall 2012.
Maverick Lin: Made it to the Round of 32 at the Kalamazoo Super Nationals in 2012. 1st singles at Highland Park as a freshman. #6 Boys 16s ETA
Michael Chen: #19 Boys 16s ETA. 2nd singles at Holmdel.
Sarthak Mohanty: #38 Boys 16s ETA. 1st singles at JP Stevens.
Robin Anderson: 2010 Junior US Open Quarter-finalist. 1st singles at Matawan. Plays for UCLA as of Fall 2011.
Thomas Caputo #25 Boys 18s ranking according to TennisRecruiting. 1st singles at Livingston. 2012 State Singles Runner-Up. Plays for Michigan as of Fall 2012.
What I'm trying to say is that there is really no point in converting highschool tennis to college format if only the first two singles positions are competitive. With that being said, there is nothing against other varsity players. My school team plays in one of the toughest divisions in NJ and is consistently ranked in the top 3 due to the depth of our team. Just because 2nd and 3rd singles and 1st and 2nd doubles aren't held by nationally ranked players does not mean the team isn't good. You cannot expect a highschool athlete to play at a D1 level because honestly, its two different ball games.
Also, funding is a big issue. Some schools like Livingston and Holmdel have well funded programs that allows them to get new courts every two years. This attracts some of the best players to join their program. An example of this is Maverick Lin. He was supposed to go to JP Stevens but because Highland Park had more "silverware" he moved and went to the Highland Park program. Therefore, you cannot blame schools for lacking depth in their programs if competitors draw them away.
To conclude, there is nothing wrong with the highschool system as it is now. Just because the whole varsity isn't nationally ranked does not mean the players don't have individual skills they can bring to the table. There is no point in converting to a college format. Also take into consideration that many good players choose not to play HS tennis because the level of competitiveness is nothing close to that of USTA National tournaments and so on.