Right. You build the rocket for the 10 guaranteed launches, then you move right on to building the next rocket, because you'll need another in case you lose the first. Then you build another after that, and that one can put up some payloads that were not-so-guaranteed. Then you build another after that, and by then, some of the labor that used to go completely into construction is now reconditioning and inspecting the reusable hardware instead. Eventually, your construction crews are building at the replacement rate for whatever sized fleet of reusable rockets you need to put up the payloads you can cram into your launch pipeline.
As it is, that pipeline is fixed at 10 launches per year. If the pipeline were fatter, they could fit more launches into it. If they can do more launches, they can court more customers.
If cost to orbit is cheap enough, you can do things like launch a big, dumb tank of water, xenon, hydrazine, liquid fuel, food supplies, oxygen candles, or anything else useful only in quantity, and requiring no life support. With those up there, you can do rendezvous-and-go missions, and it costs less money to get to wherever you're going. If you can do more launches, you can spread more missions out across multiple launches.
Consider you'd need to invest not only in manufacturing the rockets, but also on shipping them to launch sites, increasing launch site capacities to deal with extra launches, adding extra capacity for fueling the extra rockets, to prepare and mate the payloads (that'll need to be built by someone, somewhere)... Nothing there is really cheap.
In the end, the rocket is not really the most expensive part. It's just the part that, until recently, was expensive and thrown away after a single use.
Hopefully false, and you need to increase that number to at least 20, more likely 30 to pay for the investment. That is very the problem lies - the risk.
Looking at the spaceflight calendar for the rest of the year, there are 6 flights where Ariane 5 could used, if you exclude all the national missions from non-EU countries. Let's round that to 10 as this does not cover all the remaining launches. So we are at a possible market of ~20 launches a year, with the current cheap prices of SpaceX. In order to justify reusability, Arianespace/Airbus would need to plan on having a 90/100% marketshare of the very competitive market, while their US competitors could benefit from quite lucrative military missions.
As it is, that pipeline is fixed at 10 launches per year. If the pipeline were fatter, they could fit more launches into it. If they can do more launches, they can court more customers.
If cost to orbit is cheap enough, you can do things like launch a big, dumb tank of water, xenon, hydrazine, liquid fuel, food supplies, oxygen candles, or anything else useful only in quantity, and requiring no life support. With those up there, you can do rendezvous-and-go missions, and it costs less money to get to wherever you're going. If you can do more launches, you can spread more missions out across multiple launches.