Not usually a good idea to mess up with the default, and you can always middle-click or ctrl+click or cmd+click or long press. If you open in new tab by default you risk breaking user expectations, disorient them and you break users wanting to open in the same tab.
I won't be as good as the person behind css-tricks [1] and definitely not an UX expert, only interested in the topic, but let's try:
- I'm tech savvy enough and want to open in the same page: you are preventing me from doing it easily, while it's easy for you to open it in a new tab if you want it this way.
- I'm not tech savvy. I barely know to use a browser. The link opened in a new tab and I didn't notice. The back button I know to use is greyed out. I'm confused.
Said differently: you can always open normal links in new tabs. I do it all the time, to the point I almost never understand someone requesting a link to open in a new tab: help yourself, it's so easy. Don't rely on the page author to make the call for you because they can't please everyone. However, target=_blank links are harder to open in place, while it's reasonable to want and expect a link to behave normally. I get a bit mad when for some reason a new tab was opened and I didn't want to nor I expected it. Just leave me alone, you know. If I don't want to come back to the previous page, or if I want to do it using the familiar back button (or gesture), you should not force me to close the previous tab and break my history buttons.
Moreover, nothing usually tells you that a link is going to behave differently as you expected (principle of least surprise), and even if it does, I shouldn't have to install an extension or fiddle with the web dev tools to work around this and get the default and expected behavior, that's terrible UX for obvious reasons.
Deviations from the default behavior are sometimes better / necessary but you always risk causing surprise and thus confusion, which you should also avoid for good UX. Even someone who want to open the link in a new tab probably expects a link to open in place. It's usually impossible to know in advance if users will prefer opening a particular link in a new tab, especially since different users will have different preferences in the general case. As a user who will sometimes prefer a new tab, it's also uncomfortable to rely on the author of the page to make the call so you may as well just help yourself.
So, for good UX, the normal behavior is supposed to be the best option, unless in specific cases like those discussed in [1], basically when the confusion or frustration caused by the normal behavior is greater than for the new tab behavior, like loss of work or disrupted playback.
I don't know if there are proper studies about links specifically, but I'd expect existing research to prove the principle of least surprise, generally accepted as basic UX principle. I'm a bit lazy to find such work, on a phone during a small insomnia, I hope you'll forgive me. A "(opens in a new tab)" tag partially fixes the issue but doesn't fully address it.
