Defending wins championships, it's as simple as that. Better forward play can turn the occasional result, or turn a draw into a win - and every now and then there are exceptional teams that successfully win trophies through attacking - but longer-term it's the lack of goals conceded that makes the biggest difference for most champions. Especially in knockout competitions, but also in the league. Italy's Euros campaign succeeded on the back of a gritty, stingy defense - their attacking play was good but they never lost; Chelsea can't be scored upon nearly at all right now (like the Mourinho Chelsea of old) and are still favorites for the title IMO. Man Utd's best years under Ferguson always featured imperious defenders like Ferdinand. Arsenal have never been the same since their star defenders from the 90s and 2000s left or retired.
Likewise Liverpool's title season saw the front three in excellent form for most of the season but van Dijk was untouchable for large portions of the campaign, and his parters in Matip and Gomez fed off that confidence and energy to play up to their full potential. The forward-thinking fullbacks, Trent A-A and Andy Robertson, had the confidence to become deadly attacking weapons knowing that the center backs were in world-class form behind them.
If Liverpool's defensive unit can do a passable imitation of the first half of the 2019-20 season, silverware is a real possibility. And van Dijk is key to that. He was expensive but teams like Man City would happily pay double what he cost for a player of his ability right now. Liverpool still have a title-winning squad, even if they do lack depth.