I see 2 issues with your math.It really will boil down to each individuals situation. For example:
5 Line family with unlimited data. Family uses 10 gigabytes combined (Remember, this is an example, as an individual situation... I use this example because it is the same exact way my cousin's family plan was).
700 Min Family Talk Plan, Unlim TXT, 5 lines w/ unlimited data, through att. 20% employer discount.
Family Talk 700 Plan
$60 ($48 after discount) - Talk for first line
$40 - Partner lines ($10 per)
$30 - Text
$30 ($24 after discount) - Data (x5 = $120)
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$238 + Taxes/Fees
$271 after est. taxes and fees
Phones (Phones vary, for this purpose I'll use 64GB iPhone 6s' as examples since they got varying gb iPhones and I can't remember which ones)
$299 w/ 2yr + 5.5% sales tax (varies per state, obv) = $316.49 x 5 = 1582.45
$45 Renewal fee x 5 = $225
Total One Time Fees = $1807.45, or $75.31 per month if spread over 24 months
$271 (service) + $75.31 (equipment monthly from 1 time costs) = $346.31
(Add an estimated $28.5 after taxes/fees per month after unlimited data plan increase, and you'll get to $374.81 per month)
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Mobile Share Value 15gb Plan
$100 ($80 after discount) - Talk/Text/Data plan
$75 line fees ($15 out of contract per)
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$155 + Taxes/Fees
$176.7 after est taxes and fees
Phones(Phones vary, for this purpose I'll use 64GB iPhone 6s' as examples since they got varying gb iPhones and I can't remember which ones)
$749.99 over 24 months (Next 18) = $31.25/mo x 5 = $156.25
$15 Renewal fee x 5 = $75
$41.25 5.5% sales tax (varies per state, obv) x 5 = $206.25
Total One Time Fees = $206.25 taxes + $75 upgrade fees = $281.25
One time fees spread over 24 months to match 700 min plan for fairness = $11.72 /month
Total Monthly phone fees from above = $156.25
$176.7 service fees + $156.25 phone fees = $332.95 /month
Add $11.72 from one time fees and you get = $344.67
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Conclusion and notes:
700 Min Plan BEFORE unlim price increase = $346.31
700 Min Plan AFTER unlim price increase = $374.81
15gb Mobile Share value w/ Next 18 = $344.67
Now a couple things
If the family on the 700 min plan DOES NOT (Did not) upgrade every 2 years, the bill stayed at the original service fees of $271 per month, no matter what. With a mobile share value plan, the plan has the chance to go all the way down to $156.25 service fees because the phones are paid off. There are a lot of people who DIDN'T upgrade exactly every two years. Families like to do hand me downs, people keep phones for longer because they still work, waiting for whatever random new phone to be released etc etc. So this is just an example, and this is an example used from my cousin's family plan. In any case, they come out a few bucks ahead every month over the course of 24 months with the 15gb MSV, and more than a few bucks ahead after the unlimited price increase. They don't use more than 10gb a month, so for THEM this worked. Not all families work out like this. Not all families have employer discounts (in which case the MSV works out even better in situations like this).
Bottom line, is every single customer needs to do what is right for them, not what is right for some random someone on Macrumors forums.
First you forgot to factor in the resale value of iPhones.
Second you have a $2 per month price difference between 700min before and 15GB family. In other words you are within a margin of error, esp because you are estimating plan taxes. So I wouldn't say, "they are a few bucks ahead," but a "wash" instead, maybe because it could actually be slightly more or slightly less and even then it's slightly tricky.
Because going from unlimited to 15GB means if any of their usage habits change, then suddenly they could be right at that limit. So they are giving up future-proofing.
Also with that said, if that family is only using 10GB combined, then you could convert some of the lines to 2GB or maybe even 200 MB service. As which point the cost would drop to a point where it is definitely cheaper. Plus it would avoid the $5 unlimited rate increase.
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