I agree with people saying that Eurostar is far too expensive for travel for those people who do not wish to book well in advance. The trouble is that the company do not have competition. I say the tunnel should be opened up to low cost operators - such as IZY - who operate low cost high speed trains between Paris and Brussels as well as other destinations. (Italy also has a low cost high speed train operator). IZY >
https://www.b-europe.com/EN/Trains/IZY
I do not understand this "Eurostar is a monopoly, it needs competition" talk. Between London and Paris/Brussels/Amsterdam there are low-cost airlines, standard airlines, coaches, cross-channel ferries and on the Amsterdam route, the Dutchflyer. That is ample competition. If there were a genuine monopoly concern, there would have been some kind of political intervention by now, it has been 24 years!
In addition, Izy does not compete with Thalys. They are run by Thalys, who compete with itself to prevent someone else doing so, if anything it's an anti-competitive strategy. Izy uses the same staff, often the same trainsets and often the same route as the core Thalys services, which have now gained a Business class, which means higher fares on those services. Eurostar probably could do the same as Izy at some point, but it doesn't seem to really fit in with the Eurostar brand nor the direction of Eurostar's current offering.
Let's say that another operator does decide to launch a low-cost variant - there is no space at St Pancras, expensive track access fees on the whole route, unless lengthy diversions are considered, Stratford seems to be a no-goer (the station in its current format means you cannot reverse trains there, and fitting in a departure lounge for a high-capacity TGV would be a miracle) and with Eurostar cautious enough around Brexit (think the lack of Amsterdam>London directs), it's not going to get any easier.
Eurostar can improve by sorting out its station capacity, modifying the timetable, sorting out the Amsterdam service and Ashford problems and then it can offer more, another operator would not do anything to improve the overall service on the route (although DB trains to Germany might, given it'd be a new route, but DB would have done it by now if they were serious).