To state the obvious: something can be legal but immoral.
Is this the reason that Sequential has chosen to reproduce the OBX?
Same reason he decided to go with the single keyboard version of the Prophet 10 and not the more well known double keyboard/dual engine. The OBX is also likely easier to implement. It's monotimbral and has 1 LFO. The OBXa is bi timbral and has three LFOs. Dave likely wanted to just "keep it simple".
The Prophet 10 in the same box as P5 with single keyboard is the design Dave originally wanted for the P10. Back then it wouldn't work because of issues with heat dissipation, so the dual keyboard that puts the knobs into back-pain-bad-posture territory was a band-aid solution. It's human nature to look at massive old objects and be amazed or see them as somehow more substantial, but the reality is the dual keyboard design was what's known as a "kludge" in engineering.
Personally I think a P5/P10 is approaching the maximum physical size for a synth, that is feasible for a consumer base large enough to justify the product production. People might be willing to pay a fortune for a vintage 200lb Yamaha CS-80, but the reality is that if it were reissued today in original size and weight, the chance of reaching critical sales mass is non existent (and why nobody is dumb enough to try that).
Dave's cost of implementing dual-timbrality or more LFOs isn't a significant factor. He's mentioned that the Rev2 is one of his lowest-cost to produce products.
I don't have an answer about Dave re-issueing the OB-X vs the OB-Xa except to say:
1) At this point, the reissue of the OB-X or OB-Xa from Sequential is pure speculation, as no announcement has been made. We only know Sequential registered OB-X, which seems to nod in that direction.
2) If the OB-X is in fact coming and not the OB-Xa, it could be for any number of reasons. There is a general sense (maybe rumor) that older Oberheim models were generally more desirable than their successor products, so maybe he feels that the target buying audience would be more interested in the OB-X. Or maybe his decision was made based on what he already sees Uli doing, and he wanted to do something different.