[Bud]

Tumblr did fine before app stores ever existed, and could do perfectly well without them now as well. Web is enough.

[radlad]

Before app stores existed, smartphones weren't used the way they are today. Consumer habits have shifted... you'd be missing out on a HUGE part of the market.

[h0l0cube]

Missing out on a huge part of the market is comfortable territory for Tumblr

[Bud]

That simply doesn't matter. Tumblr just needs to succeed in ANY slice of the market it can find. A small slice would do nicely.

The best bet would be the slice it originally appealed to, with its original kinds of content, in the original medium it was in.