It's not much of a buff. First of all, it only applies to a mulligan on the play. Additionally, most decks that play Delver want to play it turn 1 on the play no matter what, and are probably fetching to do so. So the only way those decks would take a different line of play would be if they had a Brainstorm or Thought Scour in hand and cast it on turn 2 in response to Delver's trigger because they knew there wasn't an instant or sorcery on top of their library, in addition to all of the above conditions also being true. Or they had a G Probe that they weren't going to cast turn 1 but decided to do so to refresh the top of their deck. In other words, all of the following conditions must be met for this to have any effect whatsoever on a Delver deck's plays:

1. The Delver deck is on the play
2. The Delver deck mulligans
3. The Delver deck has a Delver of Secrets in hand
4. The Delver deck has a Thought Scour, Brainstorm, or other 1 mana instant speed cantrip in hand, or a Gitaxian Probe.
5. The Delver deck has the luxury of not fetching on turn 1
6. The Delver deck wants to cast the 1CMC cantrip on upkeep turn 2 and doesn't need to cast or hold up mana for something else on that turn.
7. Alternatively to 6: The Delver deck has a Gitaxian Probe in hand. They want to save it for turn 2 or later, but seeing a non-instant, non-sorcery card on top of their library with the mulligan scry makes them Probe turn 1 to get a fresh card on top of their deck for turn 2.

That's a lot of conditions that have to be met, and even in that scenario, Delver only gets a marginal buff. In modern, there's not even a guarantee that you'll Thought Scour or Gitaxian Probe into something that will flip Delver. You're also losing out on information by Probing turn 1 on the play instead of later, and cutting yourself off from Remand, Mana Leak, or any number of other plays by Thought Scouring during your upkeep turn 2, so it's not even all upside to make those plays. It also doesn't take into account the fact that every other deck gets slightly more powerful from this change as well. I'm not saying it's completely irrelevant; there will definitely be a non-zero number of games where this change will make the difference between a win and a loss for a Delver deck, but it's not like Delver gets significantly more powerful from this.