That's the thing. Templars aren't needed to enforce the laws.
I mean it's true that the templars acted in such a role for centuries, but over that same period of time, have taken it too far as an institution and have accepted powers and rights that they ought not have. It's not just Meredith, Alrik, Kerras or Mettin, or even Lambert. The templar order came to embody an ideology on magic that it is bad because of some ancient magisters, and they had authority over mages by divine right. As far back as Origins, it was shown, at least in the templar codex that itself said that templars were recruited mostly from the religiously fervent and less from those of integrity or character as a way to keep them from questioning their orders.
Do this long enough, and widespread enough, it's only a matter of time until most of the order is filled with ideological zealots who feel their way is the only way, and will fight to the death to ensure it remains so.
The issue with the templars is that so many of them abandoned what it actually meant to be a templar when they followed Lambert and later Lucius into a war to slaughter the mages.
If there wasn't such a systematic level of idealogy, so many templars wouldn't have left the Chantry even before Inquisition.
We need people trained and ready to enforce laws, and we likely have to rebuild the templar order or the Seekers from scratch, which is what Cassandra will do, but the templars as they stand have lost all trust of the people in Inquisition, by abandoning the role of defender under Lambert and Lucius/Envy Demon, and this is reflected in Val Royeaux by the Orlesians we can eavesdrop on.
There is a reason that the templars, in pretty much all of the endings, do not return to the status quo. It's because they as an institution are no longer trusted to fulfill that role because of their abuse of it.
Someone is needed to enforce the laws. And any force that you put toghether to do so will always be Templars. They may be named something else entirely but the Templars were always recruited from the ranks of the faithful of Thedas.
They aren't a order that was corrupted over time. They are a representation of what people think of mages.
They never, on the whole, embodied any particular ideology beyond "magic is dangerous and people need to be kept safe from it." Some were fond of quoting scriptures, others were sympathetic, others just liked hurting mages. But the tale of the Black City was always a cautionary one, not a justification for continued punishment.
And Corypheus proves even that overblown tale involving gods and thwe dangers of magic has truth to it.
It is only natural that Templars are recruited from those who will follow orders. Every militar functions in that manner. But the notion most of them were just itching to slaugther all mages is untrue. The ones who were were the ones that refused to return to Val-Royeaux after Lucius summoned them and elected instead to stay in Redcliff.
If we really want to talk about "zealots willing to fight to the death to extablish their views of how the world should be" we can include the mages in that one.
Also, the Templars return to much of the status quo in two out of three endings. Cassandra and Vivienne's with the addendum of reforms under Cass or a leash under Vivienne but their role of guarding mages who are isolated from society is restored.