The problem is cost.
It appears you haven't done a proper lander yet. You're just doing guesswork. I been there and reached 30k names max. So I know the drill.
Once you do it and use it live, you will see that at a larger number of names, perhaps some with significant traffic, you will need a serious server. And that costs. Some of the sites will drive a TON of bot traffic. You will have to handle millions of hits per day, and sometimes a crazy instantaneous load.
You'd need at least an say $150...$200 per month, good multi-CPU dedicated machine with to host anything above 30-40 k names in total. And that's IF you optimize everything very well. And yeah, it'll be easy to be DDoS'ed - piece of cake, actually. Also a great network connection, bandwidth etc.
To get this stuff done, you need an array of servers, or better an (expensive) cloud platform like AWS. You'll need to convince domainers to hand you $50 or more per month just to break even with your plain lander.
It's very hard to get this done and not be in the hole. You need to offer much more.
Actually I've seen plenty of similar services for other landing pages, such as A/B testing ones. They are all dead today. Obviously.
Edit: I had to use an 32-core server, dual CPU to handle the load of my names properly. That after optimizing the webserver, script, caching and everything. I'm not using the lander anymore cause I see no point for all this.
This is incorrect too. Any website could get a DDoS attack, and yet you see billions of websites there without the superpowerful server networks you say you require. Anyway, in case you want to prevent that you can use a service such as Cloudflare, which works as a middle server delivering all of what your true server has, protecting you from those attacks you mention automatically.
Now, to the significant traffic. Supposing you actually have a domain with 1 million daily hits, I can guarantee you that any decent shared hosting, or some cheap VPS could handle that, because you are serving only a very simple webpage, which does not require any enormous computing power, and that will mostly take time in starting the connections, rather than in sending or in processing the information to be sent.
Two examples:
1.- A node.js script can handle with ease, without any particularly special computing power, a few hundreds of concurrent connections (at the very same time). To be conservative let's say it can handle 100 concurrent connections: 100 users per second.
2.- A php script of that simplicity can be executed in a few milliseconds. To be conservative, let's say it takes 10 ms (actually something very slow for such tiny thing), and that your machine can only handle 1 thread at the same time; that is 100 executions per second: 100 users per second.
Mathematics:
1 million users in a day means, 1 million in 86400 seconds, which means around 11.57 users per second. Your poor conservative Node.js or PHP scripts can handle that average very easy, and up to more than 8 times that average, in case of a "rush second", without any issue.
So no, you won't need superexpensive infrastructure to serve a 1 page website.
On the hand, I am wondering about how you programmed your website that you required so much power to serve it. It does not make sense if it was a simple script. On the other hand, if you, for example, used some CMS or some other application you probably were requiring the extra processing power just for unnecessary procedures going on with each visitor. The simplest solution would have been to make a simple script, and if you see your hosting is not enough, use a cloud computing service to serve it: even though you would have millions of users, if your code is simple enough the costs should be small because you will use very little processing power and you will use very little bandwidth too; yet the cloud service would be ready to expand in case of a sudden explosion of users.
