Last I looked, Windows 8 and later shipped with 'Windows Defender', a.k.a. Microsoft Security Essentials. Neither WD or MSE are 'bloated', and neither require a 'momentous install'; WD is installed automatically with Win 8.x and doesn't require any kind of install on the part of the user at all. Both appear to work reasonably well. I have MSE on my Win 7 systems and WD on my Win 8.1 systems, both with Malware Bytes (which isn't bloated, either, and doesn't require a 'momentous install) as a backup. So far, no problems.
meanwhile, malware _does_ exist for non-Microsoft systems, you know. Not all that much of it, but it's there. Most of my OS X systems don't have anti-malware, but where there is a reason for it (usually a paper requirement that all systems attached to certain machines have anti-malware, period) I use Sophos. That's not a 'momentous install', either.