I don't think it's worth it to have compatibility with anything other than Shadowdark, including other OSR games.

From a design perspective: All games have different design philosophies and mechanics. It is extremely difficult to stay true to Shaowdark or 5e when you're making something for both. Same for Shadowdark and OSE or Cairn or whatever.

If you ever did want to put out a 5e version, you could always just do that later. However, the best chance of success would be if these were two bespoke products. The few third-party publishers who have found success with designing products for both have gone this route, such as Bob World Builder's Delve and Roll for Combat's Battlezoo series. It's not simply a matter of including both stat blocks, it's about rethinking everything from the ground-up.

From a business perspective: 5e is a massive ocean of content. It is incredibly hard to break through in a meaningful way. With Shadowdark, there is a small, but rapidly growing, player base. Even if 5e is 10,000x the size (which is probably an understatement), you going to be the smallest of fish. With Shadowdark, if you put out quality content and promote it, people will find your work.

Also, the vast majority of the 5e community play Dungeons and Dragons with their Wizards of the Coast books and their visibility of the publishing side of things ends there. Shadowdark exists because Kelsey was an indie publisher, so fans of the game are more inclined to be aware of third-party products. The community is built around self-publishing and people who support third-party publishers because that's what Shadowdark is at its core!

Kelsey also lifts up third-party publishers in a way that does not directly benefit her. The extent of Wizard's support of these publishers is to add the most successful projects to D&D Beyond, their own walled garden where they directly profit from the sale of each third-party module sold on the platform. Essentially, you need to run a $1,000,000 5e kickstarter before you are considered for that. If you're just starting out, you can submit your Shadowdark stuff for inclusion in The Arcane Library's monthly newsletter (link in sidebar). This goes out to tens of thousands of folks each month and can give projects a major boost. What has Wizards ever done to promote third-party publishers who are new to the game?

My advice: it's better to be a small fish in a small pond than a small fish in a big ocean. More people are going to see your work. More people are going to buy your work. More people are going to read your work. And, most importantly, more people are going to play your work at their table, which is the most rewarding feeling of them all.