It made sense in 4e because of their specific origin within the points of light setting, where all Tieflings were specifically the descendants of the Bael Turath noble bloodlines. It doesn’t make sense outside of the points of light setting though.Restricting it down to infernal heritage is one of the reasons why I've been so against the 4e Tiefling. I really liked them when they first came out in 2e, and were the Planetouched that were specifically Fiendish in general. Yes they generally do mean Devil Child derived from the German words TeufelLing.
I’m pretty sure Mearls was just wrong about Tieflings only being infernal in 5e. There was an Unearthed Arcana that included an abyssal tiefling subrace, and SCAG brought the idea into a published product via the variant Tiefling options. It seems to just be that the default PHB Tieflings are specifically infernal, with other fiendish planetouched being variants to the template, but still falling under the Tiefling umbrella.I do hope that 5e brings back all the Planetouched, they sort of with the Aasimar in Volo's Guide and the Genasi in that Elemental supplement, but they should bring back the rest of the Tiefling or whatever they name the rest of the Fiendish Planetouched if they are going they really want to stick to Tieflings are specifically the Infernal Planetouched,
It might actually be kind of cool to have a general planetouched race, with specific planar origins as subraces.and a consolidation of various other Planetouched that could be traced to origins like Lawful (Axani, Mechanitrix, Zenythri), Chaotic (Chaond, Cansin), Shadow (Shades? Shadar-Kai? Shadowshyfts), Ethereal (Shyft), Fey (Feytouched, OA's Spirit Folk), and Other Elemental (Ice/Magma/Whatever Genasi) that have popped up in previous editions.
It wouldn't surprise me, he does seem to imply they are going to bring back the "cosmic mutt" concept into the game separate from Tieflings.Just a weird thought but given how Mike Mearls takes about Planetouched could they be a separate race they have planned?