So basically most users of Bitcoin could be currently under threat of being sued by Bitcoin companies and individuals that joined DPL in the same way they might be under threat by the remaining companies. And even if they joined DPL, they may be asked to pay royalties for the use of the inventions prior joining DPL.

DPL changes nothing for most individuals that cannot and will not hire patent attorneys to advise them on what the DPL benefits are and what rights they are resigning. Remember that patten attorneys fees may be prohibitive for individuals in under-developed countries.

Also DPL is revocable by the signers (with only a 180-day notice), so if Bitcoin Core ends up using ANY DPL covered patent, the company owning the patent can later force all new Bitcoin users to pay royalties.

Because Bitcoin user base grows all the time with new individuals, the sole existence of DPL licensed patents in Bitcoin represents a danger to Bitcoin future almost the same as the existence of non-DPL license patents.
 
If you're publishing all your ideas and code (public disclosure), you cannot later go and file a patent in most of the world except the US, where you have a 1 year grace period. So we need to do something specific to prevent the publishers filing a US patent.
What we need much more than DPL, we need that every BIP and proposal to the Bitcoin mailing list contains a note that grants all Bitcoin users a worldwide, royalty-free, no-charge, non-exclusive, irrevocable license for the content of the e-mail or BIP.

I'm not a lawyer and this is not an advise of any kind. Please check yourself the DPL v1.1 and get your own idea. I'm speaking on behalf of myself, and not any company.
(http://defensivepatentlicense.org/content/defensive-patent-license)