Not sure that's really accurate of Lefty. Once he got that first major at Augusta in '04 (23rd overall W), his perception began to go more positively. Yes, he carries the same career US Open as Slammin' Sammy Snead, including that '06 meltdown, but going back at least 10 years when he won his 4th major and 3rd green jacket (Win #38), people putting much firmly has easily the second best golfer of his generation and from a pure results standpoint, just outside top 10 of all-time. Then there was his subsequent epic win in the '13 Open Championship with one of the greatest final rounds in majors history.
Sure, Johnson may be "basically half-way there," but that second half will be no walk in the park. I'd say Johnson has about a legit 5 year window to make more hay. To be clear, he already is probably no less than 4th/5th best in the Tiger Era (last 25 years). But getting to Lefty's 44 wins and 5 majors might be a bridge too far. But we'll see. DJ most certainly has a consistently high track record going back back nearly a decade in 3 of the 4 majors so he would project to have more than his share of chances. The Open Championship is one that he has struggled with much more in that time frame.