I guess I'm on the fence here. Certainly JLL had hit a brick wall commercially by the end of the decade. Had he continued to ply his old hits with the odd experiment, it's unlikely that he would've risen any higher than the oldies circuits which began in earnest in the late-60s. Kennedy and others, in steering JLL toward hardcore country, afforded JLL not only commercial success but, figuratively-speaking, clothes that fit better (stylistically and aesthetically) imo. And he wore them well.
However, Peter's point is well-taken; JLL certainly experimented less after '68, but he was required to be more artistically conservative by the standards of the country audience he was actively and enthusiastically courting. Catch-22. There aren't many popular music artists—esp from JLL's generation—who can experiment robustly and retain consistent chart success. I've always thought that mass commercial success comes at a price: at the same time one's elevated, he/she has fewer options for personal expression.