A billion reasons why he's NOT.
1. His lack of a French Open title, and thus a career slam, is the most obvious flaw in his resume. He played in an era with a few tough clay courters, but none of them named "Nadal," "Borg," or "Rosewall." The Grand Slams are far and away the most important tournaments in the tennis world, and if you're missing one or more, it seriously hurts your case. True, it doesn't stop Borg or Rosewall from being mentioned, but they sure as hell got a lot closer to winning the U.S. Open and Wimbledon, respectively, than Sampras ever did at the French Open. Sampras was never considered much of a threat on clay, and clay consists of approximately a third of the tennis season.
2. Even before Roger won 15, Sampras's record 14 Slams hardly cemented his place as GOAT. Going back to the Career Slam discussion, Rosewall had (and still has) more majors than anyone, even if we discount his amateur Slams. However, despite being mentioned in the GOAT discussion among Open Era and pre-Open Era contenders, he is rarely rated above Laver, due in large part to his lack of a Wimbledon title, widely considered the most important tournament in the game, despite his success at majors on grass.
3. Yes, he finished six straight seasons as the World #1, which is extremely impressive, but few of those seasons can be considered particularly dominant. Arguably his best season, 1994 saw Pete attain career highs in titles (10) and winning percentage (87). However, Federer and Borg both had four straight seasons winning 87% or better of their matches, and winning 10 titles or more was common place for a lot of great players.
4. Speaking of dominance, Sampras lost quite a few Slam matches to players we can rightfully call clowns in retrospect, even if we ignore his antics at the French Open (though we shouldn't) - his 1994 U.S. Open loss to Yzaga, getting demolished at the 1996 Aussie Open by Philippoussis, and another laugher at the AO against Kucera in '98.
5. His record against Agassi, his chief rival, is obviously impressive, one of the few things that makes him stand out when discussing his GOAT status. Why? We all know Federer's objectively abysmal record against Nadal, but Laver was 5-7 in major finals against Rosewall and Borg was 1-3 against McEnroe. However, in all of the cases except Sampras and Agassi, there is a considerable discrepancy in the age of the rivals - Federer and Rosewall have around five years on Nadal and Laver, respectively, and Borg is almost three years older than McEnroe. It isn't just the decline of physical skills, but also changes in motivation, what surfaces were played on, and other influences that must be acknowledged.
6. The source you provided adds very little to the discussion, unless credibility has suddenly become measured by the amount of times you can use the word "smoke" to illustrate your points.
Anyone else, feel free to add more!
FINNISH thread is FINNISH