Statistically if you had enough hashing power to more or less assure yourself a solved block before difficulty change then you could potentially come out even or ahead. Otherwise in an accelerating difficulty frame solo mining will fall behind pooled mining for most people. Luck is, of course, luck.
This is a bogus argument.
Consider: You give me a dollar. I roll a six-sided die. If it comes up '1', I give you $1,000. You only get to play once.
Is this a good deal? By your reasoning, it's not. Most people who take the deal will come out behind $1.
That's a stupid comparison as it is completely unrelated and has nothing to do with my reasoning.
The two cases are precisely the same, it's just more obvious in my example. With solo mining, just like in my deal, most people will come out behind. However, with solo mining, just like in my deal, the expected return is greater.
The difficulty change is irrelevant noise. It's based on the mistaken notion that you need time to allow the law of averages to even things out. That is not true. Playing a slot machine one time has precisely 1/100th the expected loss of playing the slot machine 100 times. If playing once is a bad deal, playing 100 times is 100 times worse. If playing once is a good deal, playing 100 times is 100 times better.