£20m us not a huge sum with the amount of money in modern football. And I would rather that went to our long term future rather than players who you can waste a huge amount of money on in transfer fees and contracts. As we proved last year it's the quality of your dealings not the absolute transfer fund value when it comes to players more Suarez less of Aspas would be good.
You are right the longer the period the more interest you pay but you are forgetting the compound affects of inflation on the principal capital. When your parents bought their house at the time it was a huge sum now the amount seems peanuts it will be the same with a football stadium.
20m a year is a lot of money for us right now, it can be the difference between qualifying for CL or not and not qualifying is a further 35m or bigger dent, so as I said, it would be a downward spiral.
I make more money than my parents so bought a better house even 25 years later, meaning if we can have success on the pitch in the next 10 years, we will be able to reinforce our world wide fan base improving revenues through mechandising and sponsorships. Therefore a bigger stadium will be more affordable and the demand for tickets higher.
Your option could mean, not enough money to buy players, no CL, no fan base growth, less sponsorship, less demand for tickets but an expandable and new stadium. I know what I would choose.