Do you believe that diseases like measles, mumps, rubella, rotavirus, etc. will be greatly reduced or eradicated due to COVID-19 lockdowns?

Since every disease you mentioned here is vaccine preventable, I would hope that there will be a reduction in the outbreaks of them, and that more people will see the wisdom of getting vaccinated and of vaccinating their kids. But I doubt that measles and mumps will be eradicated any time soon. At least, not totally.

Infectious diseases can't spread with out social contact. The problem is these diseases still exist in groups of people and will spread again when the lockdown ends. Some irresponsible parents have stopped getting their children vaccinated against these killer/life damaging infections so it is reasonable to expect an increase in infections after lockdown. This is crazy. The vaccinations are safe. The greatest risk comes from non-vaccination.

Those would all be gone if people would just use some common sense and get vaccinated. However, I am hoping that if people learn to wash their hands properly, we can reduce the number of cases of the flu every year. 

What anyone here "believes" is 100% irrelevant to reality. They might be right, they might be wrong. A 'belief' from a random user on the Internet has precisely NO value in science.

obviously yes. We're all staying at home, thus reducing contact and therefore reducing the chances of catching anything else

I'm sure there are positive side effects like avoiding other viruses, as well as less crime deaths and car accidents but depression, homeless
and unemployment increase and we'll lose our freedoms as well. 

measles, mumps, rubella are already essentially eradicated due to MMR vaccine arent they?

What I do know that only God can remove the causes of viruses as well as other forms of sufferings. The time for God’s Kingdom to take action “to bring to ruin those ruining the earth” is at hand. (Revelation 11:18) Then many features of Bible prophecy will become a reality in a restored paradise. (Isaiah 33:24; 35:5, 6) This is good news for all those who desire good health God’s way. Soon the “new earth” that John saw will be here, in which “death will be no more, neither will mourning nor outcry nor pain be anymore.”—Revelation 21:1, 4.