Some of those big corps did start out as startups. It is a very hard space to play in especially if software only. With hardware based approaches you just put the boxes inline and that is it. Note that the hardware boxes also do things like have a dual ethernet port card with a relay in it. If the box loses power then the ports are joined together so it still acts as a passthrough. (Our box also had a watchdog that had to be tickled every 2ms by software otherwise the relay would also kick in.)

The network infrastructure needs to be configured to divert traffic to the boxes which is fiddly and becomes a support load very quickly. If SSL forms part of the traffic you want to accelerate then there are two approaches either of which require even more system administration (not network administration).

Once a customer is non-trivial (eg 3 or more offices) then suddenly the sales process slows down drastically as the same product has to be used everywhere and supported by the networking and software people.

What this means is that if you are competing conventionally then it is very hard against the now entrenched vendors. If you are trying to be open source then you will find a very long hard and extremely likely to be an unrewarding slog. Many of the small vendors were out proving the space when the big corps all realised they needed an offering, and did all the acquisitions. The changes of being acquired are very slim now.