If you can get your national governments to subsidize cricket- which is a non starter btw- there is absolutely zero chance of England or Australia even maintaining current salary levels for their contracts,let alone doubling or tripling it.
England cannot sustain undecutting their domestic salaries by 10% indefinitely. As it is, their domestic cricket players make 50-80K per year, which is decent but in no way enough to sustain a 10% reduction in salary. People will quit playing FC cricket and get day jobs. Your proposal is akin to robbing Peter to pay Paul. Works in a pinch but unsustainable. Particularly with a IPL season expansion eating into the calendar for English cricket. The IPL revenue and carrot to the players on the other hand, is set to increase at a stupendous rate as the market matures. IPL has far more implied profit, since no business venture, even if it starts making profit right away, is peaking in profit or market-share right off the bat.
Ratings falling mid-season is something every sporting league endures. The opening few games of a new season invariably gets more viewership, as the fans are starved since last season, then interest peters off a bit, till the finale attracts more viewership as it is the closing stages and thus more competetive. This didnt prevent the NHL to expand their season from 60 games to 82 games.
You are forgetting that IPL is still a startup. Its been around for less than a decade. Kindly compare, with inflation costs adjusted, for a franchise-based sporting league, their revenue for the first ten years of existence to that of 15-20 years of existence. The latter figure is usually 2-3x more, even for the same # of matches/games played due to optimisation of the fanbase and the startup costs being paid off.
The IPL is already making more money than 99% of cricketing activity, despite having existed for 5 years or so. Standard business model predicts that it is only set to double or triple its income, even without expansion, in another 5 to ten years.