In fact, half of email marketers surveyed revealed that less than 20% of new subscribers engage with their first welcoming email. I rather suspect, however, that this could be because there isn't much to interact with. Most reveal the first email is a "thank you" note for signing up with, presumably, no call to action. Thus, the best advice is to not measure success by people not doing something you haven't asked them to do in the first place.
Instead of just saying thank you, important as that is, it would seem a very good idea to be up front about wanting the subscriber to do something. Welcoming emails are pretty banal and plain. We all get them, and i suspect we all routinely ignore them as a pleasantry that a brand felt it needed to send.
So why not do something a little more adventurous with a welcoming email? Why not put a poll in there, or ask for feedback on the first interaction with the brand that led to that person joining the email list? You might be bold and run an introductory offer past them, with a 24-hour code on a particular item as a welcome gift. It might be a piece of content that experience has shown achieves good interaction rates and so can serve as a softer sell for your first piece of communication with a new subscriber.
Whatever it is, it makes sense to say more than just "thank you," doesn't it? The last time you sent a thank you note to a friend, I bet there was a forward-looking statement at the end along the lines of "let's meet up before the kids go back to school" or something similar.
If you knew that a third of new email subscribers are likely to buy in the first 48 hours of signing up, you'd probably get more of a sense of a clock ticking down on their interest levels before new brands distract them.
So the research is clear. Treat every new sign-up as a very strong lead, and don't be the one in three brands that don't bother to welcome new prospects. While we're at it, don't be in the one in two who feel they should be doing far more with their welcoming "thank you" notes. You'll know your customers best and how hard you can push with a sales message, but whatever you do, don't expect interaction from a "thank you" that doesn't solicit any engagement. These are strong leads, so there's no harm in looking for some engagement while you have their attention.