Not really. If for example X=0.5 then still normally a block will be paid out 100% to recent miners - each share will be paid twice the PPS rate. But in lucky times when there aren't enough unpaid shares, past miners can be paid.
An X < 1 will further increase the time to maturity on old shares, won't it? Is there a way in a pay-once-PPLNS model to avoid an unbounded time to maturity?
It's variance, not maturity time. A miner submitting a share has a chance to get more than the PPS amount, but also a chance to get nothing at all, ever. If he does get paid, it will be with high probability in a short time.