if I get greater value from a SC why convert back to the Bitcoin. no need to quantify value the market uses subjective criteria to define it (i like it more is sufficient) but for this argument lets just say it has higher liquidity and confirmation times.
as for the question try answering it again with the new understanding you were lacking before.
Because Bitcoin is more secure, hence the reserve, or vault comparison.
Sure it might come a point where everyone wanting to use BTC as day-to-day currency will leave them on a sidechain for convenience purposes but while some transfer parties value convenience, anonymity or what not, others value security.
And Bitcoin is inherently more secure than any sidechains. For that reason it will continue to be used for high value transfer, inter-bank settlements and what not
I think the problem is you are misguided as to what people value. Different strokes for different folks. That's the beauty of sidechains