Roth was a great front man and had a great voice but their best success came after his departure.
That's a common mistake.
Combined sales of Van Hagar albums - 27 million.
For Van Halen? - 56 million.
Or we can break it down by album... 6.75 million/album for Van Hagar, 9.33 million/album for Van Halen. Best of Volume 1 contains 10 songs with Roth, 7 with Hagar. Best of both worlds contains more songs from the albums with Roth. Rolling Stone's readers picks are 8 Roth era songs with a whopping 2 for the Hagar era. The top iTunes downloads for Van Halen are 80% Roth era.
There is literally no measure by which Van Halen is not demonstrably better and more successful than Van Hagar.