Well #7, if by few years you are talking at least 10 years, then it can be correct, because at least from 10 years to now, the fares are not switchable between day and night , the meter (an electronic device verified an sealed by city authorities) is programmed with the time and calendar, so the driver just have to switch on the meter and automatically the correct fare for that time and day is displayed, NO CHANCE to change it.
Also, the cheapest taxi voucher as you said costs 16€, but is for airport surroundings (regular taxi around 7€-8€) and day time (if at night or weekends is 19.20€); for the city centre, taxi voucher is 20.40€(day) or 24,50€(night or weekends) when in regular taxi is 10 to 15€. Do you happen to know anyone who has been charged more than this amounts by a cheating driver??
In one thing we agree, an official entity is earning a lot of money with the fact that people think (like you, unfortunatelly) that Lisbon taxi drivers are among the most cheating taxi drivers in Europe.
Don´t forget the next time you came to Lisbon to tell me, and I will be glad to transport you and try to get you a different image of taxi drivers in Lisbon.
Hope this help someone