Well, I think the conversion rules need to be taken for what they are, not not taken for what they are not. Yes, the conversion rules lean heavily entirely on GM judgement, but that's ok for a hypothetical home campaign that switches from 5e to 6we. It's actually kind of appropriately thematic, as 6we rules tend to place GM judgement ahead of codifying things ahead of time (for example, who gets circumstantial edge when both sides have factors in their favor?).
Now, it's also true that SRM is unlike most home games in that there's no one GM. And therefore, what one GM says is fair another one may not. So "let your GM decide" is indeed not an ideal to go about character conversion for SRM. OTOH, I don't believe that I've heard there IS going to be any character conversions in SRM. The only comparable precedent I'm familiar with is when Season 5 and 5th edition began. You couldn't just convert your existing 4e characters, you had to make new characters for Chicago play. Yes, as Lormyr mentioned your 4e characters weren't completely retired... they just got relegated to only being allowed to play in Prime Missions. Seeing as how thus far most of the Prime Missions are written in 5e, I don't know how fast you want to convert your 5e runner to a 6we Prime Runner, anyway.