As a point of information, obtaining indefinite leave to remain used to be free until 2003, when Labour introduced fees (and increased them exponentially during the remainder of their time in office, which the Tories have continued to do). I'm not sure that simply being able to afford an application fee should be a criterion for being granted a vote.
Also until 2012, someone married to, or in a relationship akin to marriage with, a British citizen for over 4 years outside the UK, could obtain indefinite leave to enter, i.e. they could move to the UK and live here permanently (including being eligible to receive benefits from day 1) without demonstrating any commitment to the UK whatsoever, though they would have had to demonstrate commitment to their partner. Before 2003 this may even have been free too.
However in order for such a partner of a British person to apply for citizenship, they would still have had to spend at least 3 years in the UK, demonstrating commitment to the UK.
Of course in the more distant past a foreign woman marrying a British man became a citizen (well, a British subject) instantly.
Commonwealth citizens can vote in the UK because until 1962 they were basically all just British subjects. In Australia, British subjects who were not Australian citizens could register to vote until 1984, and those who were registered before then remain able to vote today.
I have a few relatives who have lived in the UK for many decades with ILR, but have chosen not to become British citizens. Generally this is because they would have to give up their other citizenship(s) and/or lose certain rights in their other country/ies of citizenship - which kind of shows that they are not fully committed to the UK. Though they are all commonwealth citizens so can vote in the UK anyway, but I believe they would choose to lose their UK vote rather than give up their ties to other countries.
The other difference between ILR and citizenship is that ILR is lost if one does not live in the UK for 2 years, although it can be reinstated if you show compelling circumstances for being away. Meanwhile citizenship can only (currently) be lost involuntarily if the government perceives you to be a terrorist, or you lied when applying.
Furthermore, when we were in the EU, EU/EEA citizens who had exercised their EEA treaty rights in the UK for 5 years automatically obtained "permanent residence" without needing to apply for anything (and those EU citizens who came to the UK before 2000 obtained PR on the day they arrived). This status was akin to ILR. I'm not sure it would be a good idea for such people to have automatically become British citizens like some posters have advocated.