Streaks are a well-know means of establishing habits. You do something every day, and count how many consecutive days you have had in a run. Each day adds one to the streak. And puts on the pressure not to break it. You do not want to get back to “0 days since the last time I forgot to do X”. Pressure to do it equals motivation. A positive feedback loop: the longer the streak, the more pressure. Until you snap.
I have built up almost 600 days of uninterrupted language learning on Duolingo. That is partly because I managed to fit it into my daily routine, so I have no difficulty doing two lessons (my daily target) every day. It is difficult sometimes when travelling, as your daily routine is different then, and I nearly lost the streak because of a timezone issue. In a way the streak is not why I’m learning, though. I have an intrinsic motivation (wanting to improve my language skills), and the streak maintenance is more for the benefit of keeping track how long I’ve been doing this.
My daughters also use Duolingo (not so much anymore, though), and one of them had a longish streak, and then lost it, as she was too busy one day doing the multitude of other things she does. That pretty much killed her motivation for quite some time. Elsewhere on the internet there are similar stories of people who lost streaks due to circumstances outside their control, with a negative effect on their motivation. Even if you still want to learn the language (or do whatever activity you want to establish as a habit), it’s a punch that you first need to recover from.
Like so many ideas, streaks as a motivational tool sound good, but they have their own problems. I don’t know what would happen if I lost my streak. While I hope it would not bother me, it still might affect my willingness to continue. I just don’t know. So here the pressure that has built up over time is clearly negative through the uncertainty it introduces. On previous occasions, streaks have helped me to maintain a habit for a limited period of time, but never as long as with my language skills.
When I was still working as an academic, I set myself a target of writing at least 1000 words every day. It went up to the point that I went to bed, realised I hadn’t done any writing, jumped out again and sat in front of the computer until I had done it. While I managed to do that for quite a while, at some point it just petered out. If you have to force yourself to do something, then streaks will not help you. My Duolingo habit supports this: I am genuinely interested and motivated to learn, and two lessons a day is an easily manageable task. The writing target I had set myself was probably too ambitious to maintain in the long run.
If you use streaks merely to keep track for how long you have done something, then they are probaby alright. But if they become your main motivational driver, then I would think they have missed their purpose. And losing your streak might well lead you to abandoning the task you wanted to habitualise.