First, I should point out that at least a third of Sprint's user-base is via in-house and third party MVNOs, IIRC (can't find the statistics currently), and SoftBank is pushing further into the MVNO territory.
Second, MVNOs aren't a
huge business and they're still more niche because people prefer to whinge about their bills and contracts, and I doubt you'd ever see those sorts of influx numbers on the Sprint network in general, let alone Ting. Ting as of mid-2013
only has 25,000 subscribers total.
Third, since Ting has a roaming deal with Verizon, even if their business grew that big in the first place to where there might be Sprint reliability issues, it might be worth it for them to negotiate a transition over to Verizon as the primary carrier.
Fourth, if Sprint can justify building out the network further to support more customers, it'll happen, otherwise they'll likely get sold off and absorbed by either Verizon or AT&T (or yet
another foreign telecom); at which point Ting might either have to transition to Verizon anyway or pull the plug if Sprint disappeared entirely.
Fifth, if it came to it, Tucows is sufficiently transparent with their Ting operations, customer friendly and responsible enough that I would seriously doubt they'd just pull the plug and go dark like a lot of smaller MVNOs do. They'll probably give notice to port out with a deadline if that sort of situation came about.
Sixth and final, Sprint coverage and reception in general already has a pretty low bar reliability wise in most areas. I'm sure it could get worse than it already is, but there's not many areas where they're top dog on coverage and quality. With expectations already set low with a lot of folks, I'm not sure it really matters. ;)
From those points reach your own conclusions.