Increasing the price of services is one way of generating revenue to grow dividends.  They can also cut costs and/or become more efficient.  However, I agree both are not entirely sustainable for long term dividend growth.  For longer term dividend growth, companies need to grow their business by investing the cashflow that they are not paying out as dividends.  To state the obvious, they either can develop new products, services, and capacity or make acquisitions that generate additional cash flow.  Using your example of Enbridge, it is critical that they keep building pipeline capacity which is supported by usage demand such as Line 3 replacement.   They also acquired Spectra and is essentially going to take all their cashflow and slowly add it to their dividend. 
When you don't hear about companies expanding to the States, or Asia, or making acquisitions, or announcing new products, that's when you have to start worrying about the dividend not growing.