Since there seems to be confusion about this, here's more or less how it works...
Each park in a chain makes a certain amount of revenue. From that revenue, the park keeps what it needs to cover its expenses. The remainder (the profit) is put into a pool that is shared throughout the chain. Now, not every park necessarily makes enough revenue in order to cover their operating expenses, and if a park can't meet their expenses they must be subsidized using the profit generated by other parks.
When it comes time to invest, parks can submit desires to corporate, but ultimately it is up to corporate how to spend the profit generated by the chain. While many enthusiasts wish to think otherwise, investments are determined based on where they will generate that greatest return on investment, not based on which park "deserves" it the most. This is why a park in a chain may get more or less investment than would be expected based on its popularity and size. It is also why one park may receive a ton of investment in a short time while another similar-sized park gets nothing.
Now, Cedar Fair has eleven theme parks. Among those eleven, Cedar Point, Knott's Berry Farm, Kings Island, Canada's Wonderland, and Carowinds generate the largest revenue. As a result, those parks receive big investments ($10+ million) frequently (generally at least once every three years, and sometimes more often). Valleyfair and Michigan's Adventure generate the least revenue, and those parks only get a big investment perhaps once a decade (or even less). Additionally, no park is going to get a big roller coaster if that isn't what the park needs. At many Cedar Fair parks, the waterpark is just as big of a draw, so large slide complexes or wave pools may be installed instead. Some parks skew toward families, so a dark ride is a better choice than a coaster. Lastly, a park is not "neglected" if they don't receive a coaster every x number of years, they are neglected if they are not given what they need to maintain their current attendance and revenue.
Looking at Michigan's Adventure specifically, the park has the lowest revenue in the Cedar Fair chain and it's potential for growth is limited due to the population of the region and the park's proximity to Cedar Point. The park does, however, have a sizable profit margin and actually performs quite well compared to some other parks in the chain, but as a major investment would net minimal return it is not a smart choice for Cedar Fair. The park receives flat rides and waterpark additions every couple years as those attract roughly the same number of visitors as a big B&M would at a fraction of the cost. Does this mean the park will never receive a coaster again? Absolutely not, and a $7 million dollar ride like a Chance Hyper GT-X would be an excellent addition for the park. However, it is pretty low in the pecking order, and at the moment there are at least five other parks in the chain (possibly more) that will get a coaster first. Therefore, the best shot for a coaster in 2018 would be to get a used ride (perhaps whatever Knott's removes for their 2018 project), as in the eyes of the GP that is just as good as a brand new ride.
Lastly, as far as RMC is concerned, they are not the answer in all cases. RMC is a good option for taking rides that don't live up to their potential and making them great. They are a poor option, however, for rides that were once great and have merely suffered from aging and limited maintenance. While I haven't ridden the ride, from everything I've heard Shivering Timbers is among the best wood coasters in the Cedar Fair chain, and therefore it would be a horrible candidate for a RMC treatment. Additionally, RMC's style is poorly suited to basic out and back coasters, as most of their elements work better within a freeform layout. Finally, in a park with multiple wooden coasters, if they are going to invest in a RMC conversion they will do it on the ride that will benefit the most from it. Therefore, if Michigan's Adventure ever gets an I-Box conversion, it will almost certainly occur on Wolverine Wildcat. However, I don't see Michigan's Adventure getting a RMC anytime soon as those rides are outside of what Cedar Fair would likely be willing to spend on the park.
Coaster Count: 535 (114 wood, 444 steel); Park Count: 110
Top 5 Wood: El Toro, Voyage, Thunderhead, Outlaw Run, Boulder Dash
Top 5 Steel: Superman the Ride, Fury 325, Millennium Force, X2, Intimidator 305