"A better player will win in a longer race"
Not true. Longer races actually favours the weaker player.
In longer races (i.e. race to 100), a lower level of concentration is sustained as nobody can operate at maximum output for extended periods (even 100m sprinters are only at maximum effort for about 10m, according to Usain Bolt). Short races (i.e. race to 5-10) requires peak effort in all aspects; shot making and strategy. In a long race the stronger potter is more likely to prevail whereas in a shorter race the better all around player will turn down a 60-80% pot for a 95% safe. The added pressure of the increased cost of errors heightens the need to make better decisions.
Discipline, strategy and execution are more valuable than in a needlessly long race where simply making fewer potting errors in a lower quality mach will suffice, so the better player, the player better able to achieve a higher level, is more likely to succeed in a shorter race.
EDIT:
Short version for those not able consider the merits of a proposition they don't immediately understand or agree with - I'm not saying a bar banger will topple a world beater in if the set is long enough. When two players are closely matched, a shorter race gives the advantage to the better player, the player most able to reach a higher level of play. A longer race gives the advantage to whoever has the best stamina, which is a very minor aspect of pool ability."
How do you figure that stamina is a "very minor" aspect of pool ability? Pool is a game of dexterity and concentration and is often contested with players having to play many matches in a single day. So it seems logical that being able to sustain a high level of concentration and dexterity so as to be able to use all of one's physical skills and knowledge at close to peak performance is a valuable trait. In fact, it is one of the major reasons that pool players often use drugs and other stimulants.
But beyond that aspect, if two players are equally fit but one is better than the other one then the better player has the advantage in races of any length but that advantage is less when the races get shorter.
The easiest way to think of this is unforced errors. The stronger player simply has less unforced errors. So while a weaker player may well be able to play five games perfectly he is unlikely to be able to play ten games perfectly. The stronger player is likley to be able to play more streaks of games without error than the weaker player. Thus your contention is simply not true. Shorter sets provide more opportunity for weaker players to play at a higher level in short bursts. Shorter sets will tend to have more opportunity for variance where a few rolls can have a bigger impact on the results. Shorter sets also more opportunity for the better player to play below his average for a number of games with little room to recover.
A long race allows for the better player to assess their opponent's strengths and weaknesses. The better player can settle down and relax while maintaining control and focus.
I regularly play races to 15 and much prefer them to races to 5,7, or 9. Quite often I have found myself behind through silly mistakes and been able to catch up and win the set when I start to really focus.
So based on my experiences I am fully convinced that your assertion is incorrect. In pool, long races favor the stronger player. And the "stronger" player is the one who has all of the characteristics that make them stronger including more stamina.