For one, it's a football term which connotes the 5 conferences with automatic bids to NY6 games. There is no NY6 in basketball.

And for your reference to the NCAA selection committee, Washington, a P5 conference champion in 2012, did not even get into the NCAA Tournament. For further reference, check the Pac-12 of several seasons ago, which got just 2 teams into the NCAA Tournament, or the SEC of 2016, which got only 3 teams into the NCAA tournament (out of 14).

2nd point: There is no "Top 5" conferences in college basketball, unless you are excluding the Pac-12 of course (tic, mostly. The Pac-12 hasn't had a national champion in 20 years and hasn't had a team in the Final 4 for nearly a decade). Counting the Pac-12, there is a top 6 though. The Big East, which does not sponsor FBS football, is no worse than the 3rd best basketball conference in the nation as currently constituted (1st before realignment), and quite possibly the 2nd best after realignment (behind the ACC).

In other words, based on the information above, it should be fairly obvious that using the term "P5" as a way to describe the haves and have not's of college basketball is distinctly inaccurate. The Big East, with current defending national champion Villanova and a best in the nation 40% (#1 Villanova, #10 Creighton, #13 Xavier, and #16 Butler) of its teams ranked in the Top 20, is clearly one of the "haves." Much more so in fact, than a couple of the "P5" football conferences.

By all means. If you don't mind being knowingly inaccurate, keep using "P5" to describe the haves and have nots of CBB. You might go ahead and do what the rest of the basketball world does, and use "Major" and "mid-major" to describe specific programs and conferences.