Those who believe in an open league system insist that the primary purpose of a domestic football league is to be as competitive as possible, because that's the only way the standard of the sport within the country can be steadily improved.
Football, being a true global sport, is more competitive than any other discipline. It's extremely tough to keep up with nearly 200 countries most of whom take the game very seriously. The domestic league system plays the role of a breeding ground for talented players in this process.
In an open league system, clubs always strive to field the best possible team; whether to avoid relegation or to win their way to the top division. This leads them to put more effort into scouting and youth development, which in turn leads to the standard of the league improving over time, ultimately resulting in the national team having more capable players and doing better at the world stage.
This philosophy is particularly applicable to India, who are trying hard to improve their FIFA rank and become a qualify for the World Cup down the line. The stakeholders in Indian football - whether they believe in open league or franchise system - all agree that the national team's standard must be improved.
The proponents of promotion also believe that in the long term, meritocracy solves the problem of geographical reach by itself. The I-League, which was initially struggling to have a pan-Indian reach, has significantly expanded its footprint on the nation's map using promotion ever since the club licensing system was implemented in 2007.
The increased emphasis on youth development created space for clubs with smaller budget but rubust a robust talent pool to be competitive at the highest level. This led to clubs from the football-rich North East India to enter the national circuits and thrive. Clubs like Aizawl FC, Neroca, TRAU, Shillong Lajong, Wahingdoh and Rangdajied all reached the top flight and made their mark. Even the North India issue was solved soon enough; Real Kashmir FC won their way to the I-League and immediately became a contender to be reckoned with (Punjab FC came via corporate entry). As a result, currently the I-League has a better geographical spread across India than the ISL.
The biggest difference the promotion method has over the expansion method is that while the latter immediately brings teams into the highest tier, the former increases participation in the lower tiers first which over time trickle-up effect on the top division. So, following the promotion method requires the league system to have patience for the process to play out over a few years. And given FSDL's past record, their inclination is towards curation for immediate results rather than letting metrit play out naturally.