- you can't just buy 15% of the hash rate
depends on who you are
Go ahead and make a list of costs and constraints (manufacturing, logistics, etc.) for achieving 15%.
Please, take into account the time necessary to build these 15% (currently 50 PH/s) from scratch; and your actual hashrate share by the time you're done...
Finally, make a list of actors that could meet those costs and actually be able to achieve 15%.
... And don't forget to account for the time and money required to build a leading semiconductor fab. And the requisite wafer manufacturing facility. Nevermind that the production of all the significant inputs of required unique equipment is watched closely and reported by trade magazines. This just cannot be done in stealth.
Yes, this is a known vulnerability. Indeed, several alts have been tanked in this manner. However, it was done with several times the previous hashrate, not a fraction.
Inconvenience, not fatal wound. OP needs to rethink the magnitudes involved.