This is a difficult question, because crypto is under continuous development, and use-cases can change.
Yes, right now, you wouldn't use Bitcoin for an instant, low-fee money transfer. However we do have developments
such as LN, which demonstrate that bitcoin
can offer these solutions. Bitcoin doesn't stand still. As crypto and bitcoin in particular grow in size and start to become integrated with the mainstream economy, new problems will present themselves, and new solutions will be developed in response.
But bitcoin isn't a one-stop-shop that covers every crypto use-case. The question of what bitcoin's role will ultimately be is open to question. Will it become digital gold, a store-of-value use case? Perhaps.
I understand bitcoin maximalism, but in many areas of life zealotry and absolutism are not the most productive approach. Are there other crypto options that offer faster cheaper payments than bitcoin does? Yes. Each has its pros and cons. Tether for example has been
embroiled in controversy, so is not necessarily the perfect option... but then nothing is the perfect option, that's the point. You pick and choose from what is available, making an educated choice.
Stable digital payments? Tether is a solid choice. You mentioned XRP, which is a great choice if you want to transfer between currencies. Maybe you want to automate some manual process using crypto, in which case you'd use a smart contract coin, most likely Ethereum. Bitcoin though? It's the original coin, still by far the biggest, still the safest (excepting stable coins) against sudden price drops, still the most solid bet for price increases... but it's not the best fit to all circumstances.
I firmly believe that crypto will transform society, and also that there will be roles for many coins. Bitcoin will likely always remain the biggest and best, but other coins can and will be a better choice for other roles.
As for the question of whether increasing use of stablecoins will boost bitcoin's price, I would say this is likely, yes, in that increasing adoption of any form of crypto will boost the whole market.