To understand in part why I-69 in southern Indiana is now a reality and why other roads in the state are being upgraded and expanded, it helps to look at the population statistics of the state.
Indiana — Population (1970–2020)
1970 5,193,669
1980 5,490,224
1990 5,544,159
2000 6,080,485
2010 6,483,802
2020 6,785,528
As the figures show, in the 50 years between 1970 and 2020, Indiana steadily grew in population, adding an additional 1,591,859 residents. That equates to a 30.65% increase from the count of the 1970 Census.
If we extrapolate these figures out another 50 years to the year 2070 and utilize the same 30.65% growth that occurred between 1970 and 2020, Indiana's population will be 8,865,292 in 2070, an increase of 2,079,764 from 2020. Bear in mind, Indiana, compared to many other states, particularly in the south and west, is considered a slow growth state. However, even at a modest growth rate, Indiana will likely add more than two million new residents by 2070.
2020 6,785,528
2070 8,865,292 (+2,079,764)
Here's the figures for same date range for Monroe County, Indiana, which was epicenter of opposition to I-69:
1970 84,849
1980 98,785
1990 108,978
2000 120,563
2010 137,974
2019 148,431 (Note: 2019 is an estimate)
The population growth for the Monroe County between 1970 and 2020 was 63,582, or a 74.93% increase from the 1970 count. Again, if we extrapolate the numbers out another 50 years to 2070 and use the same 74.93% growth figure, the county's population will be 259,650 in 2070. While the increase is not quite a doubling from 2019, it's not far off. Obviously, if Monroe County continues a similar rate of growth as it did between 1970 and 2020, bigger arterial roads capable of handling the additional traffic will be necessary. It's not inconceivable that at some point State Road 46 between Bloomington and Columbus will be expanded. State Road 37 to the south could see an upgrade as well.
2019 148,431 (est.)
2070 259,650 (+111,219)
If Indiana's population growth to the year 2070 — now 48 1/2 years away — matches its growth of the previous 50 years, it will necessitate, of course, the building of more roads and bridges. Likely this will involve the upgrading of several roads to expressway or interstate standards, and the adding of lane miles in other instances through the expansion of already existing roads. In certain select locations new terrain roads may be warranted as well. As mentioned above, Indiana is generally considered a slow growth state. For comparison, here are the population figures of two states which had similar but smaller population counts to Indiana in 1970 but in the 50 years hence grew at a much faster pace:
Georgia
1970 4,589,575
2020 10,711,908
North Carolina
1970 5,082,059
2020 10,439,388
Until the population growth of Indiana, and the United States, generally, begins to wane, road and bridge building throughout the country will continue to be robust. Because ... because there are few feasible options otherwise.
Edit: Minor edits for grammar and readability; added the population statistics for Georgia and North Carolina.