Is it permissible to sell and repair internet and computer services for someone who uses them for both permissible and forbidden things, knowing that the majority of the use is for permissible things, and that withholding these services may cause greater harm to Muslims?

The Shari'ah is built upon achieving benefits and averting harms. When they conflict, averting harm takes precedence. Something ruled unlawful might contain a benefit, but the predominance of harms renders that benefit negligible. The ruling is based on what is common, not what is rare, and acting upon strong conjectures is obligatory. What you mentioned regarding statistics on internet usage contradicts what is well-known, and there are studies that show the harms of the internet outweigh its benefits, and that a large percentage of its use involves forbidden matters, as is the case with pornographic websites. The ruling on working in this field and earning a livelihood from it has been detailed in previous fatwas, which depend on the user's situation and their knowledge of how to use it.