No, he can't pull their changes into his commercial version, and he'd probably want to avoid pulling their changes into his AGPL version. But, the hypothetical forkers would also have to re-apply their patches to pkrumins version each time he comes out with an update, assuming they want to incorporate his changes. It's close to a level playing field, with what I'd say is a slight advantage for pkrumins. As opposed to the situation he's in with any other Open Source license.

The only danger for pkrumins is if the "forker" is able to make his new fork the more popular, more advanced fork. That doesn't happen very often, and really, when it comes down to it it's just a case of being out-competed, which is a danger every business faces.

IMHO, it comes down to whether pkrumins is going to gain more (in mindshare, in community contributions, etc.) by going open source, vs entirely closed source. If he's going open source, AGPLv3 lets him minimize the leverage his source releases would give to a direct competitor, with no likely reduction in his upsides (i.e., the previously mentioned increase in mindshare, the community contributions, etc).

That's of course hypothesizing that there are upsides for him in going Open Source. Clearly he seems to think there are.