Is there any reason to use an old account?

Solution 1:

The main reason why you would want to use an old account is if you had the original Collector's Edition redeemed on your account, because having the original CE from 15 years ago grants your classic characters access to those pets.

Regardless of whether you choose to create a new account or jump through the hoops to recover your old account, I can recommend getting the App Authenticator if you have a phone that supports it. It is far easier to use and just as secure.

Solution 2:

1. Ease of use

   Having multiple accounts may become a hassle in the future if you ever return to the 'retail' version of the game. You've already had issues with recovering an old account; imagine having to do this for multiple accounts in the future.

2. Financial

   Having your characters on the same account means you can pay a single subscription for both your 'retail' characters, and your classic characters. Although you may not want to now - you may very well want to do so in the future.

3. Social

   Creating a new blizzard account means you need to migrate your friends list to your new account. Keep in mind that this list applies to all blizzard games. So, even if you don't want to return to retail, you may end up playing another blizzard game, and will need (or want) to keep your friends list in sync. Not to mention the annoyance of others having to add both your accounts to their friends list.

I did the same when hearthstone came out; I now have an account just for hearthstone, and one for everything else. Sure, it's not that big of a deal - but in the end, I needed to recover my old account in the end anyway, and now I'm stuck with two accounts which can't be merged.