And heavier, larger and more expensive to manufacture.

Sure, it's more expensive in both initial and operational cost and provides a strict superset of the functionality. This is a fairly common description of what distinguishes higher-end professional products from lower-level and consumer products.

Until digital viewfinders on camera are indistinguishable in quality (both accuracy and latency), there will be a qualitative advantage to having a through-the-lens optical viewfinders available (even if there are circumstances where it remains advantageous to disable it), which means SLR with lockup mode is going to beat mirrorless when that matters.

Eventually, sure, the viewfinder gap will narrow.