It seems a lot of people here are painting the loss of trade route income a lot more unfavourably than I've found it to be. It does depend a lot on the relative size of the empires, your size versus the size of the empire your trading with and also the sizes of each others cities. If you're in the middle game with Astronomy and could have trade routes with all other civs then unless you're in Free Religion some of the other civs will be upset with you for one reason or another. Either they won't like your religion or you'll have traded with their worst enemy or you didn't join their war or this or that...
In some games I get tired of all that diplomatic nonsense (since it seems so random), you can't please all of the people all of the time, and so I adopt Mercantilism and I just don't care. It's a predictable steady income unaffected by wars, open borders, blockades, or the other civs adopting Mercantilism. That is about the time that my cannons and grenadiers board the galleons and sail with the frigates looking for a nice lucrative city or two.
If you capture a few foreign cities they become overseas trade and get the +100% bonus. If any of these overseas cities are also large cities (circa size 20) then they are better as internal trade routes than they would be as foreign ones because many of your cities will trade with them with modest multipliers than just one city with good multipliers. With 2 or 3 large overseas cities it doesn't matter if you're in Mercantilism or Free Trade the extra trade route is weak and every city can make use of the overseas gold mines. If you can set up your internal trade routes properly Mercantilism is a great civic.
The maths is quite simple. Take a size 20 city on another continent. As an internal trade source it will give base trade route 2.0 and a +100% to all your cities, so even a size 1 city will get a 4 trade route (and most will). A coastal city with a harbour will get a 5 trade route. Any of your cities of size 15 city will get an additional +25% to combine with "connected to capital" 25% bonus and give +1 trade.
If the same size 20 city was owned by another player then it gives you only one trade route which will give +150% (for sustained peace) and +100% if your city has a customs house. So that's :
2.0 x [1 +100% (overseas) + 150% (sustained peace) + 50% (harbour) + 100% (customs house) +25% (connected to capital) + 25% (size 15)]
= 2.0 x [ 550% ]
= 11 trade route commerce.
Which would you rather have? One trade route giving 11 or 15 trade routes all giving between 4 and 6 depending on city size and whether they have a harbour. Assuming the best alternative internal trade routes would give 2 that's a gain of between +2 and +4 in each of your 15 cities and could be worth 45 trade.