Bitcoin didnt require initial investments of mining hardware, initially it was mined with normal computers. Mining hardware is more like a running cost that goes up with usage/value.
The security of the Bitcoin network isn't a function of its hashrate or the number of nodes, though these things correlate; it's a function of the cost of running it.
Regardless of whether the network is run on specialized hardware, old desktops, or TI-83s, the network is only secure so long as it costs enough to run that any motivated attacker would have to spend more money than they can leverage to attack it.
There is even a site that tells you how much money you'd need to run 51% attacks on different cryptocurrencies. (It looks like the site used Nicehash payouts as a data point but Nicehash is down/being updated so those values are no longer there, but I found an article with a screenshot from May last year.)
The security of the Bitcoin network isn't a function of its hashrate or the number of nodes, though these things correlate; it's a function of the cost of running it.
Regardless of whether the network is run on specialized hardware, old desktops, or TI-83s, the network is only secure so long as it costs enough to run that any motivated attacker would have to spend more money than they can leverage to attack it.