I suspect that the Court would view turning 'may' into 'shall' as cruel and unusual, just by different means. Generally speaking, the Court has taken the view that arbitrary and mandatory sentencing provisions, in and of themselves, removes or limits the a sentencing court from making a subjective analysis of the individual facts of the case.
It would be interesting to see if somehow creating a brand new stand alone offence of 'multiple homicide' or 'aggravated homicide' or something might have more success that trying to approach the matter from the sentencing or procedure angle. Dunno. The one advantage of the DO provision is that it more broadly based towards crimes against the person, and not a specific offence.
I would argue that Constitution is more that just legislation, it is foundational legislation. It wasn't created by Parliament or a legislature, it was created by all of them, and it would take (almost) all of them to change it. Constitutions and foundational documents are hard to change - on purpose. A Constitution that is at the whim of any given Parliament or legislature is too vulnerable to politics of the day. It might seem clearer when it is for what we see as a good thing, like longer sentences for a heinous crime, but what if they do it for a not-so-good thing? A majority government has a lot of legislative power; maybe one would like a 10-year mandate.
I would surely like to hear an alternative solution. No system of humans passing judgement on other humans is going to be perfect, and every pyramid needs a top. Would you advocate elected SC judges? Not even the election-happy US does that.
I suppose we could craft a system where Parliament is supreme, and end up with populist 'three strikes' laws where people languish in prison for the damnedest things like they have in some parts of the US, but voters were satisfied. The courts, theoretically, are to protect society from the tyranny of power.
The goal posts get moved because society evolves. We don't have debtors prison, criminal offences for homosexuality or lengthy jail time for stealing a loaf of bread anymore.