[xyzzy21]

If those who already own stock in the 49% sell, then someone else can buy up to 49% even if the 51% holder never sells.

The amount of ownership can change for one already owning 51% if the amount of stock issued changes - that changes the denominator

Finally: there is proxy voting. Normally common stock holders have 1 vote per share. However most votes are "proxy voted" where you give your vote to someone else because "reasons" - like you can't attend but an institutional investor seems to represent your position so you proxy to that investor.

Via proxy voting, it's possible to push past 51% as a minority shareholder.

Related to this, you can create an informal alliance with an institutional investor owning a larger share in the same company. This appears to be how Musk "borrowed" the money for his takeover - the money came from a large institutional investor that makes money on both ends: interest paid on the loan plus the promise of higher returns on Twitter stock. That's what's called a "can't lose investment" that no ideology can beat.

Of course, the Twitter board famously owns no significant amount of stock which is problematic but reality - this means they have ZERO skin in the game so are more likely to shirk (or risk to shirk) their legal obligations for fiduciary responsibilities. They apparently got a legal wake up call over the weekend about this however.