They all still have elite pieces, yet without the depth they get bounced early each and every year. Vegas and St Louis both won recently without top 10 players on their rosters, but with incredible depth. Tampa lost their amazing third line, how do they look now? Colorado lost their scoring depth, got bounced by the Kraken and a deeper Dallas team. The Penguins HBK line was broken up and they never recovered and was a threat again. Chicago also lost a ton of their depth pieces and was never a contender again. Common for all those teams? They still had the same elite pieces but lost their depth. McDavid and Draisaitl are still doing McDavid and Draisaitl things, just like in the past few years. Difference? They have other players on the team also pulling weight. Now they're in the finals.
Depth comes into play when the top players on each team cancel each other out, which tends to happen deeper into the playoffs and particularly in the Stanley Cup Final. Either the best players go head to head, and neutralize each other's production, or they both produce a ton and it's up to the depth to put one team over the top.
If your elite players aren't at least good enough to match the other team's top players, you're going to get killed, regardless of how good your 3rd or 4th line are.
The Vegas Golden Knights did not win the cup because of their depth. Their top players DOUBLED the production of Florida's top players in the SCF.
Mark Stone, Marchsessault and Eichel each had 8 points. Bennett, Tkatchuk and Barkov had 4 or less points.
Likewise, Ryan O'Reilly outplayed Boston's best. Hence the Conn Smythe.
Your best players have to be your best players. Sometimes that means it's a wash with the other team's best and the depth makes up the difference. Sometimes it means outperforming the other team's best. But if your best can't do either, it's going to be a quick series regardless of how good your depth is.