Some would argue that we already are a third-class city, of course. But we call such people "haters," and this question (sent in via an anonymous e-mail) is technical rather than snarky.
State law categorizes local governments (including counties and school districts) in varying classes, based on population. There are no less than nine classes of county and five classes of school district, for example. There is only one class of borough, but there are two classes of township ... Upper St. Clair and everyone else. (But I kid, Upper St. Clair. Again, these classes are based on population alone.)
As for cities, there are four classes, three of which have only one city apiece. To be a city of the first class -- or as my people call it, "Philadelphia" -- you must have one million people or more. Second-class cities, a.k.a. Pittsburgh, have between 250,000 and 1 million people. A third category is reserved to cities of between 80,000 and 250,000 that decide they want the form of government -- not to mention the raw sex appeal -- of being designated a "second class city A." The sole city in this category is Scranton.
Every other city is "third class" -- which state law defines as having fewer than 250,000 people who have "not elected to become a city of the second class A." Not surprisingly, lots of places have chosen not to be grouped with Scranton: There are more than 50 third-class cities, including the Mon Valley towns of Clairton, Duquesne and McKeesport.
What difference does it make what category you're in? Plenty. Lots of rules apply to governments in one class but not another.
For example, take the 1965 Local Tax Enabling Act, which defined what taxes local governments could charge. The act prohibited a "personal property tax" on stock dividends and other income everywhere in the state ... except "cities of the second class." The legislation that scrapped the tax in Pittsburgh was a bill that created Allegheny County's 1 percent sales tax -- and that measure applied only to counties of the second class. Allegheny is the only one of those.
Why not just say "Pittsburgh" when you mean "Pittsburgh," and "Allegheny" when you mean "Allegheny"? Call it legislative sleight-of-hand.
The state Constitution says Harrisburg "shall not pass any local or special law." But it also says "all laws passed relating to each class" of municipality "shall be deemed general legislation" -- even if there's only one municipality in a class. In other words, it's not OK to pass a law that applies to Pittsburgh alone ... but you can pass a law that applies to "cities of the second class," even if Pittsburgh happens to be the only one.
Theoretically, Pittsburgh could undergo some wrenching changes if we dropped below 250,000 people. (We have roughly 317,000 folks now, according to the Census Bureau, down from nearly 370,000 in 1990.) That might be a simpler way to reform government than, say, electing a Republican mayor. But it still won't be easy: State law notes that "a change in the form of municipal government is attended by certain expense and hardship." (So is not changing the form of government, of course.) So to prevent "a temporary fluctuation in population" from wreaking havoc, the population would have to be below 250,000 for two census-takings, taken 10 years apart, in order to lose our classification.
And that's just the beginning. As state law spells out, "No change in classification ... shall become effective until 10 years after the certification" of the population loss. In the meantime, the courts would appoint a commission to recommend changes to the government structure. Pittsburgh would become a third-class city only if city officials did nothing to act on those changes.
Granted, city officials doing nothing is a foregone conclusion -- especially if taking action might threaten their jobs. But there's another, much more likely, option. The legislature could rewrite the definition of a second-class city, so it included cities with fewer than 250,000 people. Harrisburg did this after the 2000 Census, to protect the status of Lackawanna and Fayette counties.
Still by 2030, it's at least possible we could have a whole different city ... a place like, say, McKeesport. Reformers take heart!