Unfortunately whether you are a resident or not in Spain everyone is eligible to pay some form of tax in Spain particularly if you own any assets.
There have of course been problems in the past and still today that many people are simply unaware of their financial responsibilities. Those who have chosen to keep a financial representative in Spain have been told what they should be paying and when as long as they have hired a professional solicitor and accountant. However those who have not, usually did so for the purpose of the purchase of the property, but not for the annual maintenance, as they would have set certain things up through a bank account before leaving. While this is a good idea it does not mean that all aspects are covered, like the income tax that all non-resident property owners must pay regardless of if their earn money from the property or not. This in the UK for example does not happen therefore many are shocked when hit with a back dated built up tax bill.
As paying taxes is part of the law, as is declaring assets and income, the longer it is left the bigger and more severe the fines and interest will be. Surely if not before in order to sell a property in Spain your tax record will be checked and it will show that this tax has not been paid, another reason for not leaving it.
In terms of what you will be facing if you did fail to pay it by submitting the 210 form, would be an interest increase on the amount every three months, at first it is 5%, later 10%, 15% and 20% of the amount until it reaches over a year and then in addition to the interest you will also be fined, which will depend on the tax. If the tax office deem you unwilling to pay in addition to being fined and paying interest you will also have penalties which depending on the severity of the case will be between 50-150% of the tax amount. Taxes unfortunately tend to catch up and Spanish authorities have been tightening up recently.