Er, would you rather have slow speeds, or overage charges on your shared data? Any type of shared data plan is subject to one (or more) persons screwing the others and using up all the allotment.
My opinion is a combination;
On any tiered plan (shared or individual) of a specified data bucket, if you go over your limit you get throttled (at a still reasonably usable speed), however if you wish to keep full speed data rate, you pay a reasonable price per gig to extend your bucket until the next billing cycle.
This way there is no 'surprise' overages, yet there is both the option to continue at reduced speed, or option to pay additional for full speed.
Lots people keep them more than 2 years, but not necessarily as the active phone.
Ie, hand them down to family members and/or backup devices.
And of course there is the option to sell them or trade them in for credit.