Actually mobile OS usually have better security by default compared with desktop OS. For example, Android offer Application and Privacy sandbox which limit what an application can do.
That's correct. There are not much hacks that happen because of vulnerabilities present in OS but because of negligence of people. There is no way a malware can make it to your mobile unless you download some fishy app or click on some suspicious link. Rather then focusing on security of devices alone, human vigilance is also required in cyber world (or securing bitcoins).
A chain is as strong as it's weakest link, and in security of Bitcoins the weakest link is human.
It seems your major concern is basically on human vigilance. But we can all agree that, inasmuch as human vigilance and carefulness is essential when it comes to Bitcoin security, the technological advancement in Blockchain security plays a more vital role when it comes to Bitcoin security because there are so many measures provided to enhance security even when humans are not being super careful it'll be difficult for scammers and hackers to exploit them. Let's consider the Zero knowledge proofs and the multi-signature wallets for example. If you're familiar with these technologies then you'll know that these techniques makes it almost impossible for scammers and hackers to access people's wallets.
We can equally consider development and advancements in encryption and cryptography. They are also quite helpful techniques that helps to secure the bitcoin wallet. I'm not saying Human vigilance isn't important or essential, but I stand to believe that it's not the primary as there are things that are more important.