I see this in Stack Exchange questions, I see it in blog posts, and sometimes at conferences, and it takes one of two forms:
How do I do X without a plugin?
This is how you do Y without a plugin
The answer is to remove the comment at the top of the plugin, and it’s no longer a plugin.
There’s nothing wrong with plugins, and the only practical difference between using a plugin, and putting these snippets in your theme, is that they’re stored in different places. Having a lot of plugins isn’t bad because it slows your site down, it’s bad because doing too much stuff slows your site. Putting that code into the theme won’t make things magically work better.
Instead, have lots of plugins, but small fast plugins.