The issue is that unamalgamated cities allow for segregation of wealth and resources from the common good. You see this in the U.S., where you get clusters of wealth and poverty and immense racial segregation. If you care about social inequality, amalgamation is a necessary tool to combat that.
Toronto would be MUCH better off with Newmarket and Oshawa brought in.
I don't mean this to be dismissive, but this reads like you're really not familiar with Toronto or the GTA, and applying the issues of US cities to a completely different context.
Toronto is an amalgamation of six pre-existing municipalities, and much of it is suburbia. Sure, Newmarket and Oshawa are considered Toronto suburbs now (debatable for Oshawa), but there are like seventeen more layers of suburbs before you get to "urban" Toronto, so it's not such a logical expansion. If all of the GTA were amalgamated into Toronto, you would get an area larger than New York, LA, Chicago, Houston, Phoenix, and Philly combined.
Additionally, the non-Toronto parts of the GTA, by and large, aren't just Toronto's bedroom communities. Mississauga, Brampton, Vaughan, Markham, Oshawa, Ajax, Oakville etc all have significant economic activity happening within them. Oftentimes, they have their own bedroom communities, like people living in Stouffville and working in Markham. More than half of the GTA's population lives outside of Toronto. Amalgamating the GTA into Toronto would be like putting all of LA, Orange, Riverside counties into LA.
I support that too--I think all metropolitan areas should be under one local government, though of course I'm open to a district-type structure that gives some local power.
As for racial and economic segregation, ditto. GTA suburbs aren't sundown towns formed by 1960s white flight - to the contrary, many of them are immigrant towns. Toronto itself is very ethnically diverse too (and I'm not using diverse to mean not white, I mean genuinely diverse in a way that very few American cities can top). As for social/economic inequality, there's plenty of that within Toronto's borders, but it's really not a "rich suburbs/broke city" dynamic like in many US cities.
Nevertheless, balkanized urban areas like Toronto allow for that dynamic to potentially emerge. And it's not like there are no gaps in income between municipalities; compare Oakville and Toronto for instance.
And in practice, all too often the professed "social/economic inequality" being combatted by these amalgamations is more of a *political* inequality; that is, it's a means by which suburban and rural populists can beat up on the political left's "inconvenient" vice grip on urban centres. If anything, it's more kinfolk to conservative gerrymandering than an authentic means of "addressing inequalities". (And besides, a lot of those so-called "inequalities" were already reasonably addressed through earlier forms of two-tier unitary government: Metro Toronto, the regional municipalities, etc. Which "megacities" like Ottawa are a dumbed-down version of.)
I'm not sure if we should be making local government boundary decisions based on politics. If there's a logical and beneficial reform, it should be done no matter who it benefits.
Regardless, it seems to me that the harmful effects of amalgamation on the left are blaming the wrong thing. The left in Toronto has struggled in recent mayoral elections because it can't seem to win in highly diverse inner suburban neighborhoods, often quite working class places. Winning somewhere like this should be the left's bread and butter!
In any case, the Liberals won basically everywhere in the GTA in 2021--there's no reason why at least a center-left couldn't win in a unified Toronto.
