I wouldn't say that it's really a threat. Part of what you're offering Mint is a guarantee that you aren't going to churn for an entire year each time you renew along with an up-front payment so they don't have to worry about chasing down payments. I'd also note that the $15 annual plan is a 4GB plan. Normally, Mint charges $25/mo for that 4GB plan if you buy in a 3-month increment.
It looks like Mint also doesn't include taxes and fees in their plans (See Taxes, Fees, and Surcharges https://www.mintmobile.com/plan-terms-and-conditions/). Taxes and fees can easily become a large portion of one's bill. It looks like that 12-month 4GB plan will have a $14 recovery fee ($1.17/mo) plus 2.5% ($0.38/mo). On top of that, there's the Federal Universal Service Fee (~10%, ~$1.50/mo) and whatever state/local taxes and fees exist in your area (which are probably 10-15%, but might be above 20% if you live in a high-wireless-tax state like Illinois). So that $15 4GB plan is likely around $20/mo (a third higher).
When you look at Mint's 12-month unlimited plan, it's $30/mo + taxes and fees - and it's not even unlimited! It's a 35GB plan! Plus, when you add in the taxes and fees, it's likely to cost over $38-40/mo.
Heck, on the 3-month 4GB plan, you'd be paying $25/mo + $1.50/mo in recovery fees + 2.5% ($0.63/mo) + USF and state/local taxes and fees bringing the plan to around $32-33/mo for a 4GB plan. A big portion of your savings is that you're willing to pre-pay for a year.
You can literally see that Mint values the 1-year advance payment as worth $10.58/mo to them (including the difference in recovery fees). So it's not just "Mint is $15 (really $20 with taxes/fees)." It's that pre-paid annual billing is probably worth $10-15/mo (given that I'm comparing a 3-month and 12-month plan and Mint's site doesn't seem to list 1-month plans).
> shaking my head at those who are paying double or triple that rate on T-Mobile
I think what a lot of people miss when thinking about plans is phone promos. If you don't want the latest phones, that's great and you shouldn't be on a plan offering phone promos.
For example, let's say you want a new iPhone every 2 years. T-Mobile will give you $800 in credits when you trade in your device. The trade-in is likely worth $300 making the credits worth $500. Each month you'd be getting $20.83 in credits. Let's say you're on a family plan with 4 lines and paying $42.50/line. When you subtract out the phone promo, you're down to $21.67 which is almost the same price as the Mint plan once you factor in the taxes and fees that you're paying on Mint. Plus you don't have to pre-pay a year in advance. Plus you're getting unlimited data, 40GB of hotspot, a $15 Netflix credit, 5GB of high-speed data when traveling internationally, and more. Divide that $15 Netflix credit and it becomes $3.75/line dropping the price to $17.91/line and you're getting unlimited data instead of 4GB and a cheaper price.
If you don't want fancy phones and don't want unlimited data, it might not be worth it to you. You can think of it as companies figuring out how to charge people the same money in different ways. Mint is giving you a very limited data bucket, adding taxes and fees onto the plan, and you have to buy your phone at full price. T-Mobile is offering phone promos and entertainment credits that can make the plan effectively cheaper than Mint while offering unlimited data and even international roaming.
> In terms of why T-Mobile is making this purchase, I'm pretty sure it's to take out someone who was eating their lunch.
It's highly unlikely that Mint was eating their lunch. It's more likely that T-Mobile wants a brand that can sell limited-data bundles to people like you who don't want to pay for an unlimited plan, pay for the overhead of stores, etc.
The problem with your argument is that you're comparing apples and oranges. First, you discount how much you're paying by going with the nominal charge and ignoring taxes and fees. Then you ignore the fact that you're getting 4GB of data rather than unlimited data. If I compare Mint's "unlimited" plan (which is actually only a 35GB plan), it's suddenly the same price as offerings you can get from T-Mobile (on a family plan) or its Metro sub-brand (as an individual). If you price in phone promos, entertainment, etc. T-Mobile's offerings can be a lot cheaper.
No, Mint isn't eating T-Mobile's lunch, but they have created a good following among people like yourself who aren't looking for an unlimited data bucket and want a low monthly fee and are willing to pay for a year in advance. If T-Mobile were concerned about Mint eating their lunch, they could just increase their MVNO fees. No, Mint has found a way to get people to pay for limited network access in advance which means a lot lower churn and a lot lower billing costs - Mint itself values annual billing at over $10/mo compared to quarterly billing!
Mint's rates aren't really lower than T-Mobile's. You're just interested in saving money on a limited-data bulk-purchase product that T-Mobile doesn't sell (and doesn't want to clutter their current brands with).