The market share is Windows entire problem. Linux will stay secure as long as it remains a backwater. Before Google got their mitts on it and turned it into the worlds most popular phone operating system, Android was Linux. Now you need AV on your phone.
Installing Ubuntu for a very naive computer user isn't an issue, you just show them how to to get on Facebook and they're happy - a web browser looks and works pretty much the same in any OS, but if they have a degree of Windows experience it becomes a much bigger problem. Many of my customers wouldn't know what a Windows tool was if it bit them. No disrespect to them, but there are a lot of people with lives out there who don't live to write Word documents and defrag their hard drive

, and if a computer will do what they want the OS is immaterial.
If they know and love Microsoft Office, Linux and Libre/Open Office is a non starter.
If everyone thought like you regarding Office suites, Microsoft would have abandoned Office a decade ago. I'm no fan of Microsoft and their draconian licensing is a pain in the backside, but I believe if you want Microsoft Office, you should pay for it. I don't do hooky software, company policy, most of my customers respect that, although I've had 2 in 4 years that weren't impressed. If you want Windows & Office for nowt, there are people who will entertain you, but I don't do it.