I'd bet it will be supported. A conservative estimate is that Apple sold around 12 million watches in its first year. That's less than 3% out of a potential market of around 500 million iPhone customers. For those who bought gen 1 with an iPhone 5, and liked it, Apple can't really afford to alienate them from upgrading without buying a new phone, nor can they afford to alienate iPhone 5 customers who have been on the fence. Since the iPhone 7 doesn't look as though it's going to offer too many compelling reasons to upgrade, including one big reason not to -- dropping the headphone jack, I'd say a lot of 5 users will hang onto their phones for another year.
Apple needs as many many iPhone customers as possible to enable growth of the platform, unless watch 2 will be more independent from the iPhone, and will have some minimal Android support. My guess is unless the Watch really takes off next year, Apple will further support it on as many phones as possible, for as long as possible.