with BTC just being reserved for high cost, high level transactions such as buying property, while PPC, FC and LTC can replace the actual dollar, with LTC being used for the semi-high transactions such as buying cars, etc.
There's nothing actually restricting coins to a particular use, so people would avoid exchange fees and technical complexity and gravitate towards fewer (if not only 1-2) currencies.
If bitcoin were perfect in all aspects maybe this argument would hold some water, however I highly doubt that's the case.
I think market has already spoken on this matter, just look at the money flowing into altcoins, any blatant copycat coin can get to 10k BTC market cap overnight.
Scalability concerns and the uncertainty of proof-of-work concentration are the top two issues that are eroding bitcoin's dominant position.
10k overnight could mean that value has been created instantly by copy-paste, or it could mean that people are throwing money at anything they think could make them bitcoin millionaires. I wouldn't read much into such a low cap after such a short period. The market for currencies has already spoken several times throughout history, with most cultures tending towards fewer metal backings over time.
If the reason for multiple coins is just theoretical future problems then it makes sense to have some altcoins as insurance, but to use the most popular suitable coin for everything else in the meantime. If anything, people would trade a basket currency of multiple coins, rather than limiting each coin's use to a particular class of goods. We didn't see people using tin coins for stagecoaches because they were worried about gold transmutation or El Dorado.