I must say I've not really noticed the speed upgrades that occurred over the years since the speed reached 70 Mbps, and I'm on 200 Mbps now, so I suspect you wouldn't notice any appreciable difference between 100 and 67.
Having said that do your research, engage with retentions in the right way (see link for an example below) and you'll probably be able to stick with VM at a decent rate. I don't enjoy haggling, or indeed any telephone dealings with VM, but it is an evil necessity if you want a good price. https://community.virginmedia.com/t5/Networking-and-WiFi/can-you-downgrade-package-without-using-the...
In terms of the implied unfairness of charging loyal customers more, it's hardly unfair since VM actually lose money on new customers with discounted rates. They offer discounts to acquire new customers because that's the model across the industry. If they (readily) offered the same pricing to all customers they'd go bust very quickly. There's that bit of leeway where you can squeeze them when your fixed term ends, and see if there's a mutually agreeable compromise, and if there isn't, then there's other suppliers. It is worth pointing up that there are ISPs who don't offer massively discounted introductory offers followed by end of contract rises. Most of these are smaller ISPs who focus on better customer service, such as AAISP, Aquiss, CIX, IDNet, uno, Zen Internet. But the relatively small size of these companies compared to the majors tells you that most people choose up front discounts and respond to heavy marketing, and that although almost everybody will agree "I'd pay more for better quality service", in reality few will.