It's all debatable, but underground is probably the best option on most dimensions apart from cost, but its outrageously expensive so trade-offs are required. Elevated makes sense in this context and gives many of the same benefit at an expected lower cost. Of course, "expected lower cost" is doing a lot of work here - we don't know the actual cost of the downtown elevated section, as it hasn't had detailed design completed yet. An elevated line can be all three of these things: much cheaper than tunnel (1), outrageously expensive (2), and subject to cost overruns and delays (3).
I think that's the bigger picture takeaway here - we have to fix the cost escalation crisis of transit projects in Calgary (and Canada overall), and find ways to make these things cheaper and simpler while still being fast, efficient and high capacity. This is beyond what the Green line can solve as it's a problem that's been brewing for too long everywhere. It's systemic to the industry, not project-specific.
IMO, much of the back-and-forth on the Greenline over the years was important but ultimately myopic engineering approaches to the problem (e.g. adjusting alignments, station design) while failing to tackle the primary drivers in cost escalation that has more to do with things like political interference, long-term funding availability and predictability, procurement processes, standardization (or lack thereof), regulations and design standards.
