I think OP argument is why evenually a self founded wide empire would catch up.
In practice on Civ V on a standard map size on standard speed it doesn't because of:
1. start up costs for a new city (generic ones).
2. Extremely fast time to modern era compared to Civ IV and before.
3. The specific to Civ V requiring yet another copy of buildings for national wonders, especally science ones.
4. When going tall, the large decrease in startup costs of the first 4 cities with Civ V Tradition.
5. New to BNW, the very large power of food cargo ships (and to a lesser extent caravans), which also appear to benefit the capital when running tradition more than it would several cities when running liberty.
#1 & #2 are so critical, that I think on Huge map on epic speed a 6 city wide Liberty empire is faster than 4 city tradition. (Bigger tech costs from huge map, plus longer game from epic)
In practice on Civ V on a standard map size on standard speed it doesn't because of:
1. start up costs for a new city (generic ones).
2. Extremely fast time to modern era compared to Civ IV and before.
3. The specific to Civ V requiring yet another copy of buildings for national wonders, especally science ones.
4. When going tall, the large decrease in startup costs of the first 4 cities with Civ V Tradition.
5. New to BNW, the very large power of food cargo ships (and to a lesser extent caravans), which also appear to benefit the capital when running tradition more than it would several cities when running liberty.
#1 & #2 are so critical, that I think on Huge map on epic speed a 6 city wide Liberty empire is faster than 4 city tradition. (Bigger tech costs from huge map, plus longer game from epic)